Nosfera's husband Brahm, a vampire, is killed by being stabbed in the heart with a cross. This is lampshaded.
Schlock Mercenary takes this to heart, especially Kevyn, who wears antimatter grenades on his shoulders. One of those antimatter grenades is capable of slagging a tank, and the other has the yield of a good-sized nuke.
Schlock himself runs around with two sawed off anti-tank multicannons and an oversized plasgun. The multicannons are especially ridiculous because he can hook up feeders and, quote, "fire breacher rounds on full auto."
One of the most popular books with mercenaries in the Schlockverse, The Seventy Maxims of Maximally Effective Mercenaries, contains several appropriate quotes on the subject, as shown in the quotes section for this trope. The one that quotes this trope practically verbatim can be found in poster form.
Actually subverted on one occasion, in which Schlock mocks two enemies for aiming anti-tank weapons at him. They tell him that they're not worried about a little overkill, to which Schlock responds, "OK, but you might want to use your anti-tank weapons on those tanks behind you." Incidentally this wouldn't have been overkill, he can't be killed by explosions if his friends find all the pieces.
Schlock's immune system is itself a form of overkill, it allows him to survive practically anything, even including foreign nanites.
Once, the PD Fleetmind launched an all-out assault on an entire star system as a diversion, to extract Tagon's Toughs with minimum collateral damage. To quote the UNS Marine prize crew guarding the Toughs: "'Minimal collateral damage' and 'entire star system' do not belong in the same sentence"
The fishing boat captain in Schlocktoberfest 2005 has an array of buoys that, at full power, will detect fish clear across the planet, while boiling away the oceans on their side of the planet.
In this strip, Schlock tells us that the plasgun can be used to send 5mm blasts that can travel up a sniper's scope into his brain without scorching the furnishings. This came as news to everyone, given that, as Kevyn points out, Schlock has never willingly used any setting below "vaporise the target and set the room on fire".
Riff is, in general, a very enthusiastic supporter of this trope. Regard "party favors".
The Order of the Stick: Familicide. Nothing says "overkill" like making Black Dragons the newest member of the Endangered Species list. Just how far past the spell ended up that is a sobering reminder of just how badly people underestimate the concept of OVERKILL.
The Last Days of FOXHOUND: Vulcan Raven's take on the matter: "Subtlety is a thing for philosophy, not combat. If you're going to kill someone, you might as well kill them a whole lot."
In Quentyn Quinn, Space Ranger, the Empire of the Seven Systems responded to the (admittedly rather brutal) slaughter of one of their colony ships by incinerating an entire solar system.
Black Mage's preferred form of attack - he has been known to vaporize forests, and at one point destroyed a city by accident when demonstrating to its mayor how he would destroy the city if he didn't get what he wanted. He has refrained from using spells in battles where his life was at stake because he viewed anything that wasn't complete overkill a waste of his time.
"Don't say another goddamn word. Up until now, I've been polite. If you say anything else—word one—I will kill myself. And when my tainted spirit finds it's destination, I will topple the master of that dark place. From my throne, I will lash together a machine of bone and blood, fueled by my hatred for you this fear engine will bore a hole between this world and that one. When it begins, you will hear the sound of children screaming - as though coming from a great distance. A smoking orb of nothing will grow above your bed, and from it will emerge a thousand starving crows. As I slip through the widening maw in my new form, you will catch only a glimpse of my radiance before you are incinerated. Then, as tears of bubbling pitch stream down my face, my dark work will begin. I will open one of my six mouths, and I will sing the song that ends the Earth."
At the end of the Bloodsuckers Are Not Sexy arc of Fafnir The Dragon, Stephanie Meyer comes up with the brilliant idea of inventing time-traveling mermaids. The first one she runs into is... decidedly less than friendly.
A.R.I.E.L.: I am the Automated Response, Investigation and Extreme Liquidation unit, also known as A.R.I.E.L. In the future, you, Stephanie Meyer, will instigate Lack-of-Judgment Day, in which nearly every mythological creature is melted down into plastic goo and repackaged as teenaged heartthrobs. Meyer: Oh, that sounds romantic! (targeting computer currently highlighting Kill It with Fire) A.R.I.E.L.: On behalf of what few intelligent creatures remain... *opens fire* #$@% YOU!!!