How can I de-spawn mobs in Minecraft multiplayer? [duplicate]

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.

In Minecraft 1.8 or newer, you can use the command /kill @e[type=Zombie] to kill all zombies in the world. You can also relpace Zombie with whatever mob/entity you want and it will kill it.

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,
spawn-monsters=false.

Restart the server, then if you wish, stop it again and set difficulty and spawn-monsters back how they were.

(Granted, this assumes you have file-system access, but since you're the admin, you might)

Turn gamemode into peaceful! it makes no Hostile mobs!