Preventing Java from creating new processes

Question

I am setting up a little service to allow people to run temporary Minecraft servers on a spare box that's not being put to much use.

The .jar that will start up the Minecraft server will be uneditable, as in they can't upload a new .jar and run that in its place. However, the .jar will be the Bukkit Mod/Wrapper for Minecraft servers, which allows the loading of plugins that are in the form of .jars. What's stopping someone from creating their own plugin and using it to spawn more servers?

How can I prevent someone from using a server plugin to create another server process? I am already planning to make it so that each server will be ran under it's own user account, which wont be accessible to login from.