[ubuntu] Log out/kill a user's GUI session via SSH?

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http://ubuntuforums.org – I am sure this very question has been asked around here quite a bit, but I have been looking for days and I am still unable to find a solution. I am running Ubuntu 12.10 Desktop on a Lenovo H520s. Right now the computer is used for two things: Minecraft and Minecraft Server. Now, I'm sure that running the server software and the client software on the same machine is probably not the most ideal thing to do, but I'm trying to work with what I have. I have created a user account specifically for running Minecraft (the game) so that it runs at least somewhat independently of the server software (Hardware)