There are many ways of ridding oneself of an unwanted cohabitant. I suggest a staged approach, using increasing levels of persuasiveness.
A good starting technique would be to follow your roomate around constantly, shouting incoherently at the top of your voice. You will find that after a while they will become disconcerted and leave.
If this fails, you would be best to go on the offensive, and use somewhat more proactive techniques. Try hiding under their bed with a power drill until they go to sleep, and then start drilling up through the mattress and into their back. Laugh manically while you are doing this to improve the effect. This should soon see them off.
A small minority of people will put up with even this level of discomfort in order to avoid moving, and if your roommate should turn out to be one such individual, I would suggest that you adopt the only course of action which is, in my experience, absolutely guaranteed to permanently remove them from your life.
First, incapacitate the offending individual by strapping them securely to a radiator (the radiator can, if you so wish, be left on to increase discomfort). Then, tie a piece of sausage to a long synthetic line, and force feed it to them. After nature has taken its course, the line will emerge from the other end of their digestive tract and you will be the proud owner of a roommate on a string!
Bundle the roommate on a string into the back of your car, and drive to the nearest airport. Tie a long rope to the string through your roommate, and ascertain which aircraft will shortly be leaving for Mogadishu. Sneak onto the runway and covertly tie the rope to the back of the plane.

Your roommate will in this way be towed behind the aircraft to a completely different continent, and you will be free of them forever.