I've rejected quite a few guys. It didn't give me an ego rush, in fact it made me feel sorry and a little upset, but I still prefer that to having a relationship with someone who makes me feel uncomfortable just because he wants it. And believe me, guys have many ways to make a woman feel uncomfortable. For example -to name the latest case-, by saying "you can't decide whether or not we're compatible until we've walked together for a while. And by 'walked together' I mean 'had sex'".
As for how I reject them, I try to be as polite and subtle as possible, giving only the details he asks for and trying my best not to hurt him. This usually leads to long explanations, in the last case even by e-mail. In that case I even tried to suggest what kind of woman would be best for him. I even tried the "it's not you, it's me" speech once, which led him to tell me I'd turn 40 and still be single, virgin and surrounded by cats.I put up with it and he got a new girlfriend less than 2 weeks later. The only time when I had to repeatedly tell a guy to stop calling me - and even told him that he was crazy - was when the guy started harassing me with phone calls several times a day.