This guy is lying about me sleeping with him=(?

A friend of mine is telling all his mate's that I slept with him. This is not true. He also said that I keep asking him for more. That is also not even close to the truth I cannot stand the twit. The other worst part of this is one of his mate's I actually like. When I go to the pub they always stare at me and ask why I slept with David. The guy I like will not even talk to me now because he thinks I am hooked up with his best mate. I tell them I did not but he has them convinced that I did. How the hell do I convince the guy I like that I did not sleep with his best friend? Also, I have never been known as someone who just sleeps around so i do not understand why they seem to just blindly believe this meat head=(.