In short, a weaponized Idiot Ball.
One of the most effective ways to deal with an enemy is to rob them of their greatest weapon - their mind. This trope is a subtrope of Mind Rape, an intentional attack on someone's intellect. In layman's terms, deliberately making someone stupid.
This trope can take many forms. It can be temporary or permanent. It can be achieved through magic, surgery, or weaponised Phlebotinum Induced Stupidity. It can have any number of degrees of severity, ranging from simply taking the edge off the victim's reasoning to full-on infantilisation or even reducing them to a drooling vegetable. Finally, depending on the work's tone, it can either be Played for Laughs or be a potent source of Nightmare Fuel.
This attack also has a variety of uses. It can be a means of control, a way to neutralise a troublesome Guile Hero or Chessmaster, or even a particularly sadistic torture method or Fate Worse than Death (especially if the victim's aware of what's happening to them). Some characters, in fact, use it to boost their own knowledge and intellect via a form of Vampiric Draining, though this is not strictly necessary to the trope. Whatever the case, though, removing some IQ points from the target must be a deliberate purpose of the attack, not just a side-effect.
Contrast Idiot Ball, Plot-Induced Stupidity, and Took a Level in Dumbass, where it's the writer dumbing down their characters. Compare Phlebotinum Induced Stupidity, which is often unintentional and possibly either self-inflicted or the result of some environmental effect, as well as Brought Down to Normal or Depower, an event with a similar purpose used to strip away someone's physical or supernatural abilities.
During the fight with Pana of Giga's Six Cyber Knights in Bobobo-bo Bo-bobo, Poppa Rocks/Don Patch summons a literal idiot ball in retaliation against Pana's Psycho Balls. Later on against Poet, he stabs a spike into Poet which injects the latter with a fluid that makes Poet think just like Poppa Rocks, and given how Poppa Rocks (Hell, just about any of the fighter on the good side) think, this doesn't really go well for Poet in regards to his techniques.
One of the main subplots of Identity Crisis revealed that certain members of the Justice League frequently used Zatanna to mind wipe villains if they ever found out things like their secret identities (or in the main story's case, to prevent Dr. Light from trying to rape the Elongated Man's wife again).
One Super Mario Bros. story from Valiant Comics involved a new weapon from King Koopa- the Stoopid Bomb, a hair-trigger explosive that can make anyone (Except King Toadstool) mind-numbingly dumb. There's also the Smart Bomb developed to counter the Stoopid Bomb's effects, though they all get used up on one Snifit. By the end of the story, everyone, including Koopa's army, are rendered idiots, and the now-genius snifit's attempts to take over are ignored.
A medical lobotomy is used for this purpose in One Flew Over the Cuckoo's Nest to get rid of a particularly troublesome patient - the protagonist, in fact.
Likewise, the impending use of a lobotomy to achieve this end hangs over Baby Doll's head throughout most of Sucker Punch. The original purpose is to make it impossible for her to give testimony about her stepfather's crimes, though, rather than to make her stop causing trouble in the institution.
HAL: Dave. My mind is going. I can feel it. I can feel it. My mind is going. There is no question about it. I can feel it. I can feel it. I can feel it. I'm a... fraid. Good afternoon, gentlemen. I am a HAL 9000 computer...
When Megamind visits the monument to Metro Man, he reminisces about old times and brings up an "Illiteracy Ray", a rather specific variant on this trope.
In Rocky and Bullwinkle, the villains plan to release Really Bad Television, a series of tv shows so horrible that anybody who watches them becomes a mindless idiot. As per usual (see the regular series below) Bulwinkle is unaffected.
John Coffey does this to Percy Wetmore in The Green Mile, not through evil intentions but to remove the threat to his friends, resulting in the guard ending up in an insane asylum. It says a lot that Coffey only does this to people he considers to be pure evil.
Also, whilst all cases so far of it doing this are accidental, mind magic has considerable potential to be used in this way. This is because (a) mind mages tend to rapidly devolve into insane, amoral monsters with little regard for the people they control, and (b) mind magic inflicts permanent brain damage on its victims. Even when Molly first uses it with the best of intentions, the fact that she has baggage with the person she uses it on leads to deeply ingrained paranoid delusions that could end up being permanent.
In The Wheel of Time, several characters have realized that warping the World of Dreams to make their opponents stupid can be a disturbingly effective attack. When Mesaana attempted to use this against Egwene, her defense was so strong that it broke the Forsaken's mind, reducing her to an infantile mental state.
Used as a main plot point of an entire arc in Perry Rhodan, when extragalactic invaders did this to the entire Milky Way Galaxy. Some rare few people were immune (including most of the protagonists), but civilization basically broke down right then and there because those few were usually too busy trying to keep all the morons in check and from accidentally killing themselves and each other to keep things running on the necessary scale, let alone go after the actual threat; the need to figure out what was going on and find a countermeasure drove much of the early plot. (The scale of the event was later justified in that the so-called 'Cosmic Swarms' were revealed to have been created by Sufficiently Advanced Aliens with the intent to help spread intelligence on a cosmic scale. This particular one had simply been hijacked by ambitious underlings who decided to Reverse Polarity...)
This is an ability of the Oversoul, the millions old AI that monitors humanity in Orson Scott Card's Homecoming series. To prevent humanity from developing to a point where it can potentially wipe itself out, the Oversoul can make those who delve into forbidden ideas suddenly stupid or forgetful until they move on to something else. It takes a lot of willpower for a human being to overcome this. This ability is so powerful that at one point the Oversoul does it to itself.
Discussed in Isaac Asimov's first novel, Pebble in the Sky, which revolves around an intelligence-enhancing Upgrade Artifact. At a point, the villain threatens to Reverse Polarity and use the machine to reduce the heroes to vegetables.
Duke Roger, in the Song of the Lioness books, put magical dolls of various people who might suspect him under a veil to make them unable to really suspect him - as soon as they intended to do something or seriously investigate, their resolve would fail.
Topher does this to the head of Rossum Corp. in the finale, using the magic mind-wipey gun to give the villain Active architecture and turn him into a doll against his will. Hell, he's doing this pretty much in every episode, in some form or another.
Also, the Bad Future shown in the two Epitaph episodes is the result of this being done on a massive scale. By "massive scale", we mean somewhere around 80-90% of the world's population having their minds wiped in a single instant and then being reprogrammed to mercilessly hunt down and murder anybody that wasn't wiped.
Basic D&D Immortals could perform Ability Score attacks against an opponent's Intelligence.
The Yellow Musk Creeper reduces the Intelligence of its victims.
The Feeblemind spell massively lowers the target's Intelligence.
Psionic attacks, from Original D&D (Eldritch Wizardry) to AD&D1.
The spell Ray of Idiocy, which reduces the target's Intelligence by 1d4+1, can be hilariously broken if you know how to use it. How? Well, having a 0 in any stat is debilitating; 0 Intelligence causes a coma. Now, most animals have Intelligence 2 or less, and many magical beasts aren't much brighter. Thus, this spell can one-shot anything from a housecat to the Tarrasque.
Ego Whip could cause idiocy when used against a defenseless psionic.
Psionic Blast could cause a Feeblemind effect on a non-psionic creature.
Magic: The Gathering has Nemesis of Reason whose attack causes you to discard 1/6 of your deck, representative of your mind/knowledge/power. And more generally, this is the idea behind the alternate win condition of "milling." (usually associated with blue mana, which represents mental magic, among other things). When you have no more cards in your deck, you lose. This path to victory is represented by such wonderful spells as Mind Funeral.
Champions. The Drain power (and in older editions, Destruction and Transfer) can be used to lower another character's Intelligence, either temporarily or for a considerable time.
In Mutant UA, this exists as a psionic power known as "Parasit" (would be "Parasite" in English). The character increases one of his own attributes by stealing it from someone else, and Intelligence can be the chosen attribute.
Warhammer Fantasy Roleplay 1st edition had the 1st level Battle Magic spell Steal Mind, which temporarily reduced its victims to a drooling vegetable capable of doing little more than gibbering and eating grass.
In Black Crusade, this trope is one of the possible effects for a Daemon Weapon. The weapon is described as having some incorporeal parts, and the attacks go right through a target. Then they become too stupid to breathe.
Warhammer Fantasy has a special rule called 'Stupidity' which causes you to have to take an LD test or just wander forward. Several armies have magic weapons (such as the Wood Elf 'Dragontooth Arrows') or abilities that cause a wounded model to be subject to the special rule, playing this trope literally. On the other hand, since it's on a model-by-model basis, and most models only have 1 wound, it's usually a wasted effort, but hey.
A Harlequin weapon in Warhammer 40K is called the Neuro-disruptor, which "uses intricate psycho-crystalline circuitry to emit a wave of particles that disrupt the brain's neural pathways, turning the target into a drooling imbecile or killing them outright".
In The Elder Scrolls there are spells for reducing other people's attributes; one such spell affects Intelligence. Such spells can either be cast or used to enchant items. Thus, a weapon could be designed to make the enemy more stupid each time it hits.
NetHack's Mind Flayers have an attack that reduces intelligence. They can also cause amnesia, making you forget which of your items are which.
In Shin Megami Tensei: Strange Journey, Zelenin's Song is supposed to be so angelic, so beautiful, that anyone who hears it cannot help but lose all their will to fight and submit immediately to her. Which is rather creepy in and of itself, but then the angels get the idea of weaponizing it to subdue corrupt mercenaries, brainwash demons, and eventually assimilate all of humanity.
4X game Ascendancy includes the Intellect Scrambler, which temporarily erases the memories of the crew of the affected ship. The in-game description claims that it can "turn an experienced crew into a bunch of bumbling rookies".
In Portal 2, Wheatley is an Intelligence Dampening Sphere who was created as part of GLaDOS's AI, to prevent her from killing everyone in the facility by giving her a constant stream of terrible ideas.
In Zak McKracken and the Alien Mindbenders by LucasArts, the villainous Caponians are using a stupidity-inducing signal over the phone networks to slowly reduce the humans' intelligence, until we're stupider than them and they can finally invade. At the end of the game, mankind reclaims its brainpower and usher in a new age of enlightenment, with telepathy and dream-sharing replacing telephones and two-headed-squirrel burgers becoming a new fad.
A closer interpretation comes in the form of attacks such as Supersonic, which causes the Pokemon afflicted to "hurt themselves in confusion".
Fallout: New Vegas DLC Old World Blues features this as a central part of its plot. The Thinks Tanks, essentially a cyborg Brain in a Jar, are a group of moral-less scientist who threatened to destroy the already post apocalyptic world with their experiments. One of the Think Tanks, Dr. Mobius, realizes this but fails to reform his colleges, so makes them into idiots by reprogramming their robotic parts. Their stupidity serves the dual purpose of preventing them from creating more scientific hazards and preventing them from figuring out how to disable the radar fence, a feature that prevents disembodied brains (or de-brained bodies) from leaving (the think tanks think its protects the facility). Mobius then starts to play the role of an Evil Mad Scientist just to distract them so that they don't figure out that they have been made idiots, he even convinces them (without directly messing with their minds) that his robo-scorpion army is equipped with a stupidity inducing attack.
A weapon in the DLC, the Glove of Mobius, actually makes your NPC opponents stupid by reducing their intelligence stat.
Deus Ex: Human Revolution: The CASIE Aug works like this as it uses pheromones to reduce the target's logical thinking ability and directs them towards actions which will benefit you, if you convince them with the right words as well.
Inverted (with the possibility of it being played straight discussed) in Kevin & Kell, where, in one major story-arc, the Bird Conspiracy turns out to have an 'Intelligence Beam' with a... reverse setting. This results in one of the cast being threatened with the loss of a few dozen IQ points, and allows another (one of the denser members) to experience a bout of Flowers for Algernon Syndrome.
Girl Genius: Post-Villainous Breakdown, Zola announces her intent to administer a... rather crude version to Tarvek.Specifically, coring out his brain with a sword in order to make him her 'zombie slave'. In her defense, she was incredibly high when she came up with the idea.
Eerie Cuties had an epidemic of bimbofication thanks to a powerful artefact falling into the hands of a very perverted possessed doll.
Likewise, a poorly timed "wish" in The Wotch made all non-blonde girl bimbos.
This is how the Light Warriors planned to defeat Chaos.
Fighter himself qualifies, as just being around him for long periods of time can drain your intelligence, according to Red Mage.
In a Narbonic non-canon Sunday strip (actually a dream of one of the readers, that the author decided to illustrate) an enemy hits Dave with an Intelligence Extractor Ray to incapacitate him. Instead, dumb!Dave reveals "a Forrest Gump-like ability to observe things and turn up at the right place at the right time" and saves the day.
Near the end of the series Helen invents a serum that shuts down the parts of the brain that make someone a mad scientist, permanently turning them into a normal person. She briefly considers weaponizing it, but decides to destroy it instead.
Jimmy Neutron uses this on himself as the entire basis of one episode, when he decides to be less of a genius. In the third Jimmy Timmy Power Hour the enemy gets the edge by robbing Jimmy of his genius intelligence (and Timmy of his fairies). He wasn't as stupid as in the other example, but aside from his lack of scientific know-how he also acted much ditzier than usual, forgetting common words and such.
In Futurama, an invading alien race of brains can sap intelligence away by their presence, which makes it easier to assimilate knowledge and destroy planets. It doesn't work on Fry, however, because he has an unusual brain quirk (which, implicitly, explains his general lack of intelligence).
In one Kim Possible episode, Drakken uses "Silly Hats" to avenge himself on the members of the Cerebellum Ultra-Smart Super Genius Thinking Society for not letting him join.
Appears twice in The Adventures of Sonic the Hedgehog. In "Grounder the Genius", Hacker creates a "Stupid Chip" as a decoy for the one with the Super Genius Program he stole from Robotnik. When Robotnik thinks he's recovered the Genius Chip, he ends up using the Stupid Chip on himself, with predictable results. Another episode had Robotnik inventing a Stupid Ray, which became the episode's MacGuffin: a crazed general thinks he can use it to make an army of brainwashed soldiers, and sleazy salesman Wes Weasley figures it will make it easier to sell people his worthless junk.
In the episode "NanoZIM", Zim attempts to use this on Dib by shrinking to microscopic size and destroying his intellect from within. "I'll just go to your brain and delete the knowlege of where you hid the master disk! And, as an added bonus, I might as well make your entire brain... nn-not smart... no more."
In "Plague of Babies" Zim does this successfully—he fends off a horde of aliens who are fully sentient but visually similar to human infants by combining GIR with a power amplifier to create a lobotomizing wave that destroys their minds. He then returns them to their covers as real babies to random parents (where they had already been hiding as oddly unaging babies for 8-years). Yeah Invader Zim is kind of disturbing that way.
In the Lilo & Stitch: The Series episode "Spike", Jumba reveals that one of his earlier experiments had quills that contained a venom which reduces anybody's intelligence to 1% of it's original functionality. Strangely, it doesn't seem to affect him, but he later clarifies that it does, except that even 1% of his usual intelligence is quite formidable compared to the average person.
Mermaid Man and Barnacle Boy have the "Orb of Confusion", a literal Idiot Ball which turns anyone in its range into a drooling idiot. Unfortunately, the range of the Orb is only a few feet, so it only affects the holders.
According to the production material at least, Wreck-Gar packed a ranged weapon that temporarily did this.
In the Super Friends episode "Around The World In 80 Riddles", The Riddler sprays Batman, Robin, Wonder Woman, and Superman with his "Stupid Spray", which slowly saps its victims of their intelligence. He then leads the heroes on a riddle and booby-trap filled chase around the world, stalling for time until the spray completely takes hold. At the end, the heroes become reduced to toddler-like intelligence, but manage to grab the antidote and defeat Riddler.
On Doug, Dr. Klotzenstien drained the brains of his fellow classmates to get the answers of a pop quiz correct. He tried to drain Quailman's, but ended up with a frog's.
Common in erotic fiction, where the process, often referred to as 'bimbofication', tends to be treated as a good thing by both its users and its victims.
Getting hit hard enough in the skull can cause everything from concussion, memory loss, to this trope and all the way to vegetable state. No wonder the helmet was invented!