I haven't really "rejected" anyone in the sense that they directly asked and I directly answered no, but I have had to tell two different guys that I didn't want to be asked. I had my first boyfriend a few years ago, and while I was pursuing him another friend of mine was becoming...closer. He hadn't asked me out yet, but I was beginning to notice his attention shifting toward me, and our mutual friends were noticing too. I had thought about going out with him, but I decided my feelings for the guy I was pursuing were stronger. I didn't want to reject him by way of starting a relationship with another guy, so I got alone with him one day and just made it clear that I wanted to stay friends. He said he understood, and we remained regular friends. If he felt any further pain being around me from that day forward he didn't show it, and didn't make a point of avoiding me for any length of time.
The second guy began to make his feelings known a couple of months after I broke up with my first boyfriend. He had begun inviting me to his place alone, and was making comments that suggested he liked me. I didn't see him very often, about once a week or so, and even then it was at a school club we both attended and there really wasn't any way to talk to him alone without making a scene. I had tried to respond to his comments in a way which said "Thanks, but no thanks," but he never got the hint so I guess I was never clear enough. So I ended up doing the most cowardly thing ever and explained how I felt over Facebook. I had no feelings for him at all, though I still felt really bad about doing it that way. But, it just got to a point where I couldn't drag him on any longer, and the only way I could turn him down without making a scene in public or inviting him to an awkward "rejection lunch" was to explain it over Facebook. He said he appreciated knowing for sure, and like the other if he felt any pain at being around me after that, he didn't show it to me. And trust me, I looked. I really hate disappointing people.
And now I have my current boyfriend, who I've been going out with for more than a year and a half now.