Why not just make life for him there miserable? Cook up some stinky fish every night, eat lots of beans, and play really loud techno music. What would happen if you had the electric company shut off all service to the house? Then go rent a generator and use it to power just your part of the house. Or warn him that you will have all utilities shut off on the final day of his contract and he'll be out of luck. Why not "accidentally" leave your RO/DI unit running and flood his room only? I'm sure there are many ways he can be coaxed out of the house if he won't leave willingly. Just cross your fingers and hope that he doesn't start vandalizing the place. Of course, i would only resort to this after exhausting all attempts to resolve the issue amicably.
Just a thought.