I have tried everything, but it seems my fort is invincible.
I was going to abandon my fort as I forgot to keep an eye on my animal population and they bred into the hundreds. Instead of just erasing all my hard work, I thought I'd see if I can leave the place the way I found it, desolate.
I thought I'd start with the infamous tantrum spiral as everyone was friends with pretty much 1/3 of the fortress by then and I hoped that would be enough. Half a fort slaughtered and only a few tantrums.
Unfortunately for me those tantrumers destroyed most of the mechanisms and levers of my torture devices and flood chambers, at least to the point where they were no longer usable. Wanting to gain greater momentum I continued killing with what I had in hopes of ending this within a season. Even then, my dwarves were unhappy at worst. As a last ditch effort, I recruited everyone I could and drowned them all in the last remaining flood chamber.
I did catch a break and had a champion go berserk, slaughtering 8 people, but even that was short lived. For some reason, he was confined to the floor where he started berserking, and that floor didn't get many visitors. Eventually another champion came and effortlessly flung him against the wall. That wasn't enough to kill him, and so he spent his remaining days hobbling with broken legs and face after his victims, never again to know the joys of manslaughter.
Since then, my fort of 3 guards, 1 mayor and 8 kids have survived on their own for over a year. My 3 guards rip apart the multiple sieges without breaking a sweat, and I can't really build any traps or such to kill them with (not that I'd want to waste time doing so anyway). Everyone is upbeat and happy, seemingly having forgotten the dozens of friends each they have lost during the previous year. Even the food doesn't run out because the mayor has taken it upon himself to forage.
I need some ideas on how to end this, hopefully within the year. Also post general methods to keep dwarf/pet population in check or just ways to quickly and efficiently slaughter many things, especially w/o massive preparation like having a doomsday lever that flattens the entire fortress.