I also don't want to date either a doormat or a stalker, but I have no idea what that has to do with anything.
I'm not suggesting that you're obligated to date/reciprocate or entertain the guy's fantasies. Just let them know whats what and don't encourage that kind of unbalanced relationship. Props if you can pull that off and give feedback about why you wouldn't bang him without giving him an impression that he has a chance or attempting to destroy him.
My suspicion for the last part of that is becoming a wing-girl for him and helping him to see that there are other girls out there.