scorned by another server

If you REALLY think they deserve it, you can do something really laggy. I can suggest something, but remember: With great power comes great FPS drop... or something like that. Anyway, there's a neat little trick that you can do to ruin the frame rate of anyone within render distance. I'm talking less than 1 FPS if you're willing to do a lot of digging. First make a big diamond-shaped hole in the ground 29 blocks across in a high-traffic area. Make it as deep as possible. Next, cover it all up except for one block at the exact center. Make sure this tiny hole is the only way for sunlight to get in. If you've done it right, placing or removing solid blocks over this hole should cause a drop in FPS. Next, pillar up into the sky (about y=240 should be fine) and begin building a redstone clock to make a sticky piston move a solid block back and forth in the column over the hole a few times every second. Activate it, then log off in a location from which you can easily deactivate it.

Players who wander within render distance will experience a horrific FPS drop. The only trace of this device will be a 1x1 hole in the ground and a small structure close to the build limit. Unless players know exactly what's going on and what to look for, this will be nearly impossible to disarm without MCedit. I've noticed that controls seem to go out of sync with these devices nearby. Pressing the spacebar twice doesn't make players fly in creative mode and 1x1 pillars are impossible because of the tricky timing.

I don't think it will wreck the entire server because the strain seems to be client-side, but that's a good thing because a beefy server does not negate its effects. If you logged off right next to the controls you will be able to hold that entire region for ransom. For example, you can offer to turn it off if the offenders say "Pretty please with a cherry on top" over voice chat. Note: You are pretty much guaranteed to be banned after this shenanigans.

An alternative design is a diamond 59 blocks across with a hole in the middle built way up in the sky. Use the same clock-and-piston design to create the lighting updates. It's a LOT less work than digging a hole, but it's easy to spot because of the giant shadow. That said, there's not much they could do about it.