Ugh. I once was set up with this guy who was the biggest nerd, like think Napoleon Dynamite, all the way. Though he was a nerd, he wasn't super astute about picking up hints as to the fact that it -wasn't- happening between us, like, ever. Finally, I couldn't dumb down my hints any more and had to just tell him, "Look, it's like this - me and you, it's not going to happen. Like, ever." LOL. I felt kind of bad and yes, I felt like a B, but with some people, especially stalkery people like the guy you're talking about, you have to be absolutely clear from the get-go.

Oh, and get some pepper spray. Generally not a bad idea for women anyway.