Flirtation or the beginnings of a Stalker?

What makes for a certified stalker? I mean, when a flirtation becomes classified as stalking?
I work with a guy that has been borderline obsessed with me since the day he started working here. He flirts and I admittedly can't resist a good complement so I usually flirt back and have fun with it.

This guy has worked here for at least 6 months and does this to me every single day, all day long. I'm not afraid of him although when he first started working here, he seemed to pop up out of nowhere and I can only imagine he must have been waiting for me. He no longer does that plus there's usually plenty of people around and everyone knows the way he is with me.

I've told him flat out that it's never going to happen between us. Still, he IMs me all day long and I do mean all day long, telling me how beautiful I am and how he won't stop until he has me. He's even gotten down on his knees and proposed.

What do you make of someone who does this? I mean does he really think that his persistence is going to work? I really don't know what to make of him any more.