no longer applicable for minecraft versions beta 1.6 and later

While playing on a Minecraft SMP bukkit server, I tried using the /thor command to manipulate lightning on a few pigs with my pickaxe.

But it seems that I can't kill it, I even tried putting them in a pit and hitting them with a diamond sword, to no avail. However, they still make the damage sound.

I even tried using the /butcher command provided with WorldEdit, but it still would not work!

So, how do I kill the zombie pigmen?

EDIT:
I know that restarting the server, unloading the chunk on the server etc. will despawn the mob, but what I want to do is to kill it. So,

1. Is this a bug?
2. Does it happen in other servers?
3. What can I do to fix it?

This is a glitch with Minecraft that it also spawns multiple pigmen from one pig. The easiest way is to disconnect and reconnect. They should disappear.

This has happened in my server.

And it IS a client side bug like SevenSidedDie said.

I had the same issue with invulnerable pigs appearing client-side only from a chicken spawner. Dis- and reconnecting makes them vanish. This is a bug, you cannot kill them.

Try getting everyone on the server to leave the area, and come back. The easiest way I can think of to achieve this if there are a lot of people playing is to try a server reset. This should cause them to despawn.

Edit As I've found out on my own server, this is purely client-side. Logging out and back in should make them disappear.