As far as I know, I don’t give off any hints that I would be receptive to any advances. On the other hand, I’ve been hit on in a grocery store and at a collector car auction. Both times it took me by surprise as I was engrossed in what I was doing and not even looking to interact with anyone. I have no idea what is in people’s minds.
Regarding Facebook, the situation has not come up as I don’t accept friendship requests from people who appear to be suspect. Keys are: recently joined Facebook, no or very few friends, no personal information or personal information that doesn’t make sense, etc.
Anyway, sound like you need to cut the guy loose…