In an episode of Ben 10, a giant tick-like alien lands on Yellowstone Park and begins sucking all life in the area dry. The Tennysons are in the area, but are unable to stop it immediately due to the usual aliens proving ineffective. By the time Ben finally learns to use the new alien of the week, the entire area is gray and dead, the usual geysers are spitting poison, and the ground is brittle and breaking apart in large floating chunks. After the tick is destroyed, the background literally turns green and lush again in mid-conversation, and the gunk that Ben has to wash off the Rustbucket is the only evidence that the tick was ever there. Why then did the gunk remain? Just to make life suck for Ben.
In an episode of DuckTales (1987), a magical golden duck artifact with the power to turn things into gold has unleashed a magic wave that is turning the world into gold. Scrooge and the guy who accidentally started the whole thing are rushing to return the artifact to the shrine fountain it came from. They eventually reach the shrine just ahead of the magic effect and throw the duck into the water even as they're turned to gold too... and then everything is returned to normal.
An episode of My Life as a Teenage Robot involved Jenny being turned into a rampaging monster by a tiny machine that had infected her. As soon as the machine was removed all the changes were undone in seconds right before the camera. The thing that makes this particularly egregious is the fact that Jenny is a robot, but then again, so was the thing that infected her in the first place (done by the Cluster.)
The Gilligan's Planet episode "Too Many Gilligans" featured an alien cloning machine that began cranking out copies of the cast until the landscape was filled with them. When the machine was destroyed, all the clones vanished.
In Superman: The Animated Series, this is true with Mister Mxyzptlk's powers. When he's banished back to his home dimension, everything he did is instantly undone; most notably, victims of Baleful Polymorph are restored to human form. (Although Lois, who he turned into a horse, still wants to eat the carrot he was feeding her, and questions why when she notices.)
The Adventures of Jimmy Neutron: Boy Genius once had an episode where Jimmy used his Hypno-beam to make his parents think that the next day was his birthday in order to get a chemistry set... instead, the Hypno-beam's power makes them think that everyday is his birthday. After eventually getting sick of it, Jimmy decides to unhypnotize them, only for the party clown to take the Hypno-beam and accidentally destroy it in a misperformed party trick. The next day his parents tell him that (assumably because he's had so many birthdays) that he's now 18 and going to college, but they were just faking it and destroying the Hypno-beam really made the hypnosis end (the bill for the clown was still there, though).
In the episode "Chaos in Crystal", the Monster of the Week, Rex Shard, transforms large swathes of the desert near Megakat City into crystal, as well as many kats, the prison he broke out of, and the water in the Megakat Reservoir. Transforming Shard back into a normal kat fixes it all, except possibly for the Warden, whose crystallized body was clearly shattered, but that was left unresolved. When one takes into consideration the fact that showing a crystallized being's body turn back into scattered flesh and blood would be something not meant to be shown in a cartoon series for young teens, It's almost Getting Crap Past the Radar, as it was most certainly fatal.
Most episodes involving Dr. Viper and his mutations are also included. Anti-mutagens apparently are "reverse mutagens" and capable of shrinking bloated, mutated creatures back to normal, rather than stopping further mutation.
In a Treehouse of Horror episode based on Bram Stoker's Dracula, in order to de-vampirize Bart they have to kill the head vampire. (It doesn't work, since the vampire-slayers turn out to be vampires themselves, and in this universe vampires apparently benefit from ontological N-Word Privileges.) They also didn't kill the real head vampire. The episode ends before the only human (Lisa) can, so it's never revealed if it would have worked.
Zigzagged in another Treehouse of Horror episode, when a witch turns Homer into a mishmash creature. Killing the witch undoes his transformation... except for his chicken undercarriage, which he can conveniently use to lay eggs to feed his family.
In the first season finale of Transformers Prime, Unicron unleashes a whack of natural disasters all over Earth, including loads of tornadoes and a humongous tidal wave that rises above the cityscape. After he's been defeated, the wall of water plainly freezes mid-air and promptly falls apart, and the blowing winds also disappear.
In the episode "Secret of My Excess", Spike's greed causes him to grow into a larger dragon progressively over the course of the episode, until by the end he is a rampaging dragon the size of a small mountain. When he realizes that he was wrong to be so greedy, Spike immediately, and magically, poofs back to his original size.
Even after the spell breaks completely, the changes Rarity made to Ponyville still remain. It takes three princesses all day to reverse everything.
Part of why Gang leaves Crystal Cove at the end of Scooby-Doo! Mystery Incorporated. After they destroy the Evil Entity that has manipulated everyone for hundreds of years, everything it ever did was erased from history. All of the cases and all of the monsters are now gone. Furthermore, this means everyone that the Evil Entity's curse would've corrupted got to live happy, healthy and productive lives (especially the gang's families); the entire scenario weighs heavily on them since they were five of the only six or so people on the planet who still remember the original timeline. As for the other person, it turns out to be Harlan Ellison of all people (who is able to remember multiple timelines due to being a "genius") and has become the new Mr. E. He's invited them to study at the new university he is at and will grant them plenty of new mysteries to solve.
Happens all the time in Martin Mystery; whatever effect has had a monster on humans (like turning them into zombies) will be completely reversed once the monster is beaten/captured/banished. Another example: in one episode, a geode was used to bring dinosaurs back to life. But after the geode is destroyed, the dinosaurs it created immediately disintegrate.
Invoked (sort of) in Goth Kids 3: Dawn of the Posers: Henrietta is apparently brainwashed by emoplants bent on world domination, causing her to switch cliques. When the others confront the plants they discover that the whole thing was a scam and Henrietta only became emo by the power of suggestion. Afterwards they let Henrietta save face by pretending that "killing the plant leader" freed her from their control.
Played Straight in Pinkeye. A newly zombified Kenny turns the other inhabitants of the town into the walking undead, prompting the boys to fend them off with the use of chainsaws. Kyle is informed killing the original zombie magically turns everyone back to normal, which it does, but that's tough luck for everyone who was already hacked to bits by then.
On Steven Universe,Onion uses a Replicator Wand to flood the entire town in millions of cheap plastic toys, and then replicates cars to throw at the Gems. When they finally get the Wand back Garnet destroys it, causing everything to disappear.
Subverted at the end of the "Diamond Days" arc. Steven successfully convinces White Diamond to join her "sisters" Blue and Yellow—plus Steven himself, who carries Pink Diamond's Gem—in healing all of the corrupted Gems on Earth. By combining their powers, they're able to permanently restore the Gems to their pre-monstrous forms...however, most of those Gems were corrupted for thousands of years, and they retain qualities of their beast forms even after being brought back to their original selves.
In the final story arc of Gravity Falls, the demon Bill Cipher and his otherworldly minions have invaded Gravity Falls and transformed it into a chaotic land. In the series finale, Bill Cipher is destroyed, causing his extradimensional minions to be sucked back into to their own universe and everything Bill did is reversed.
Samurai Jack; In Season 5, Jack meets an assassin named Ashi, whom he eventually redeems and starts a romance with. In the penultimate episode, she is revealed to be Aku's biological daughter, and after briefly being subject to a Villain Override she uses her new powers to send Jack back to the past to kill Aku. However, a little while later she realizes that since Aku was destroyed in the past, she and her sisters were never conceived in the future, and she fades away to nothingness in Jack's arms.
While "The Collector" has one instance of trope aversion, every single episode of Ladybug other than the first episode of her two-part Origin Story plays the trope more or less straight, as all the damage caused by the super villain of the week OR Ladybug and Cat Noir is undone when she tosses her Lucky Charm in the air.
However that's specifically Ladybug's power to restore everything. Most of the time even after the Akuma is reverted and purified the effects of their power still sticks, until Ladybug using her restoration power. The one major exception is Scarlet Moth in the season 2 finale. When his weapon (which was boosted to allow him to create infinite Akuma's instead of just one) is broken, all the akuma are instantly purified as well as anyone affected by their abilities like Darkblade's enslaved knights, even though this usually requires Ladybug to personally fix
Space Ghost episode "The Sorcerer". The title villain changes Jan and Jace into giant versions of Blip (the Team Pet monkey) using the powers of his magical wand. When Jace grabs the wand away from him and breaks it, the spell is broken too and Jan and Jace turn back to normal.
Averted in the episode "Marco Jr." of Star vs. the Forces of Evil. Destroying a painting that was causing Marco's body to transform doesn't stop the process, as the ash that the painting became still has the magic enchantment on it. Star and his parents find themselves having to take a personality test for him in order to overwrite the spell instead.
Played a bit straighter in "Conquer." Meteora, formerly known as Miss Heinous, has been rampaging through Mewni, shooting people with her newfound laser eye powers that take people's souls, leaving them in a lifeless state with black eyes, with her growing bigger every time she does so. Seeing this happen before at the hands of Toffee, Star tries to get to the sanctuary where there is a substance that can revive everyone it's happened to, but fails. In the end, after Meteroa is turned into a baby, everyone's souls are automatically restored and they revive, saving Star from having to make loads of trips there.