Tell the person that you don't think a relationship will workout with them. (Hopefully this isn't a friend or someone you see often.) Say you will be too busy when summer rolls around or something lame like that. Or start blowing off their phone calls. The person will hopefully get the idea that things are done with!
Good luck
Edited to add: if you are in the actual act itself and you just want it to stop, complain that it hurts or act like your leg cramped up or something. That should stop the moment! If not, yell "Get the hell off me!"
