Talk to his supervisor, since this guy won't leave you alone and ignores your pleas. Let the supervisor know its affecting your work due to the distraction and unwelcomeness. Hopefully, if the supervisor is on the ball, he'll talk to the offending party and let him know what he's doing is not ok. This might throw some fear into him, if he knows a supervisor is watching, or at least aware of the situation. If the offending party has any brains, he'll know that if he doesn't stop, further complaints may mean discliplinary action against him.

Chanteyranger