How do I turn a guy down?

This guy had a huge crush on my friend a couple years ago and he was a huge creeper, like he wrote her this love note and semi-stalked her for a while. I don't think he's THAT bad anymore, but he likes me now. I just am getting vibes that he'll ask me out soon. He flirts with me a lot and has asked for my number and such. How can i turn him down? the only guys i get asked out by are the guys i don't like. i don't want to seem b*****. the last guy, i just wanted to be nice, but i ended up blowing him off completely because he just kept asking me on dates and i would say i was busy (douche bag move, i know) and he hated me for a while. i want to be really straightforward this time, but not too harsh.