The roommate, if not on the lease, should not be living there to begin with; it could be considered a breach of your lease agreement with the landlord. The roommate could be asked by you or demanded by the landlord to leave immediately. Since the roommate has no leasehold rights, you might simply wish to ask this person to leave. Period. If you think this person has some sort of leasehold (like a month to month) by simply living there, then give the person one month notice in writing to leave and if the person doesn't leave, you might need to seek eviction of this person through the court system or see if a civil matter police escort can help you (local police or Sheriff's department).