Most Helpful Girls
I have had someone like that, more then once really
If they don't get it when you clearly say it to them (without been nice at that point, just saying them not going to happen not interested" then do your best to simply ignore them, at best give them the most general stranger response (If there in RL if its in the virtual realm its better to just completely ignore and even block)
It huts its sad and a pain to do it, but some don't give you any other option
well simply just tell him that you are not interested him the way he is in you and if he is a reasonable person he should understand and get off of you
Most Helpful Guys
Be extremely clear, almost rudely blunt, and if that doesn't work ignore him. It's the kindest thing you can do.