How would you get rid of an obsessed admirer?

I really thought I liked this guy but he turned out to be a big liar...that's another story...anyway, now he's obsessed and I can't seem to get rid of him. does anyone have any suggestions on how to subtly get him to back away...i don't think he's the most stable guy either...I don't know if I've turned him that way or if he was already like that but I can't deal with it. What can I do to totally turn off the obsession without triggering any serious problems?