What is the best way to tell a guy that you just want to be nothing more then friends?

I have known this guy since we were both in dippers & because of this he is under the impreshion we would make the greatest couple... needless to say he is 2 years younger then myself (but he is CRAZY tall) and we are on two different levels of machurity: he flirts with me and because he flirts he thinks that means I'm "his", I don't flirt back or anything but he still will go out of his way to impress me. I DON'T want to sound snobbish or prissy, but I am real sick of his behavor!!! At first it was nice to be noticed, but then it got anoying and unnessisary when he would seek me out to flirt (and it's never "cute" flirting). My best friend was with me last time I saw him, she was compleatly floored with his actions. And it's not like every-other encounter with him is flittachious... every time we see each other he flirts with me (even if he has a girl friend or not). I know "I just want to be friends" is the worst line EVER to say to someone, but I don't want to hurt his feelings.