I have never, in a year and a half of being on OKCupid, gotten a response to my rejection where he called me rude, fat, ugly, or whatever (though I remember that happening in my 20s when I turned guys down in person. I just figured they were drunk or stupid). On OKC, I just don't respond to assholes or guys I'm not interested in. And I usually block them, so I won't get any more messages from them. I suggest you block them after rejecting them.
Sometimes I do engage in a little conversation with someone before I decide he's a "no." In that case, I tell them I'm not interested in a much gentler way than saying, "No, thanks, I am not interested in dating you," which does sound a little imperious or harsh to me. Maybe it just looks that way in writing. I do try to acknowledge the guys for something and wish them well.
I just went through some of the messages on OKC that I still have, and I have let them down easy with these statements:
"Thanks for the kind words, but I don't really think we'd be compatible. I wish you luck in your search."
He did not answer.
"You seem very intelligent and passionate about the things you believe in. I am seeking romantic partners, but I don't think you and I would be compatible, for a few reasons. But I wish you well in your journey!"
His answer: "Thank you for checking out my profile and blog and for the kind words. You are probably right on the compatibility assessment, that said wouldn't mind being friends or shooting the shit once in a while. Good luck on your journey as well."
This is someone I went on a date with:
"It was nice meeting you the other night. I enjoyed your company and appreciate your having come all the way uptown to meet me. I didn't get the sense that we "sparked" for pursuing a romantic relationship, but I'm open to friendship if you are. If not, that's cool, too - it's all good. I wish you well & hope you meet lots of fun women here on OKC."
BTW, I never say I'm open to friendship if I don't mean it. I'm not giving out booby prizes. His answer: "Thanks for reaching out and getting together with me. Sorry it didn't work out, and wish you the best of luck with _____ and moving into the next phase of your post-divorce life."
Now here are a few where I got rejected. One that I didn't mind at all: "That's very sweet of you to say that. Thank you. Though I don't think we're a romantic match, it's nice to read your profile as well. Happy hunting!"
One that felt icky to get: "I appreciate the note, but I think I'm going to pass. Happy New Year, and good luck."
The difference between the two was that the first complimented me on something and seemed to put thought into it. The second one, even though he wished me good luck, sounded condescending when he wrote "I think I'll pass."
I think, if one is going to answer and reject someone, to do it as nicely as possible. Either that, or why bother answering at all?