Steal his lungs.
Don't worry about being "nice" because rejection always hurts--just be clear. You can be polite and you can be kind but don't hint about it, say it directly and clearly:
If/when he asks you out again, just say, "I'm sorry. You seem like a terrific guy but it just doesn't seem to click for me. I think it best to free you up to find someone who can appreciate you more than I can. I'm going to be seeing other people and I hope you will, too. Best of luck." Then do not allow it to turn into an argument; this is NOT a debate, just you delivering some slightly uncomfortable news. If he protests and argues or asks you what you don't like about him, don't debate or explain. Just repeat, " I don't want to see you again; It's just not right for me." You are not required to give a reason. Just be polite, reasonably kind, and very clear. He got along for years before he met you and he will get along fine without you--in fact, he may get along even better because we are all better off not wasting time of those who do not or cannot appreciate us fully. Cut him loose so he can find the right person. Let him know he is OK, just not right for you, no more, no less.
Just be honest and tell him that you aren't ready for a serious relationship right now. If he's a big boy, he should be able to deal with that.
So, you want him neither as a friend nor a love interest. If that's so, then just tell him that he's a nice guy, but you don't like nice guys. If you don't want contact with him, then don't worry about it.
So, if you would like him as a friend, then give him the deadly line: "I like you as a friend, but not more than that." I also liked Chiquita69's answer.
fun date, but i just do not feel the chemistry.
if he can not take a hint, move up to **** off, ******.
then a swift kick to the crotch or mace usually does it.