I ask the owners of all the servers I go to, and none of them tell me how to de-spawn mobs. I'm the only admin in my server, and there are a lot of magma cubes. I want to know how to get rid of them.
In Vanilla Servers
There are no commands for this. You could try unloading the chunk by leaving it and come back to see if they are still there (reloading the server while away from the chunk could help also.) Or you could just simply change your game mode to creative by doing
/gamemode (c, 1, or creative) to spawn items, then just kill all of them with a diamond sword or whatever you please.
In Bukkit Servers
There are many useful plugins for bukkit that allow for simple mob killing. WorldEdit and Essentials allow for the use of
/butcher which kills all aggressive mobs (maybe all mobs) around you. If you are unsure if a plugin has a mob killing command, just read the help page in the server or find a website with all the commands and their uses.
Similar to what James said, you can get rid of them by restarting the server on peaceful.
Stop the server, then swing by
server.properties and set
difficulty=0 and for good measure,
Restart the server, then if you wish, stop it again and set
spawn-monsters back how they were.
(Granted, this assumes you have file-system access, but since you're the admin, you might)
There is no way to "de-spawn" but a simple
/killall command should kill any mobs in maximum render distance.** It will end up like this:
/killall Killed ###
However, the second you do that normal mobs (zombies, skeletons, creepers) will spawn in to replace the killed ones. If those magma cubes spawned naturally then you can't do anything but continually kill them.
** = mobs killed by
/killall will not drop any experience orbs or items.