How do I get someone to stop trying to befriend me?

I just naturally prefer being alone to being with/talking to other people, but this guy at my school keeps trying to make friends with me. I'll be honest, I don't like overly-friendly people. I think he feels bad that I'm always sitting alone or something. I don't want anyone's pity though, because I like being alone, and it's kind of frustrating that he always tries to make conversation with me when I don't want to. I probably sound rude, and I know he probably means well, but I don't want or need anyone to try and be my friend. I have friends already who get me, when this guy obviously doesn't. So, how do I get him to stop talking to me and trying to be my friend?