Before you even say a word, think long and hard about why you don't want to date him/her. Be sure about your decision.
If you STILL want to say no,thank this person (if you hung out together) for all the good times you had.
Be clear about your decision, but do not be bluntly honest about the reason. "I'm sorry, but I don't think we're compatible," or "I don't feel there is potential for a relationship here," are answers that are far preferable to "I don't find you attractive," or "I feel that you are boring," which will unnecessarily hurt his/her feelings.
Do not say "I'm not ready for a relationship," or even "I already have a boyfriend/girlfriend,". This may lead the person to believe that you would date them, so unfortunately they may pursue you even more.
You are not obligated to discuss or provide a lengthier explanation of why you do not want to date someone. If the person persists, be very clear that this is your decision, that you are moving on and that they should move on as well.
If you already had a friendship, you may ask if you can still be friends. Beware though, that if you maintain the "friend" thing, he/she might not give up so easily.
That being said, don't be rude while turning him/her down, either!
Remember that he/she will be somewhat hurt, so don't be rude afterwards.
Try to end the conversation on a positive note, or steer to a more ambivalent topic.
If you know someone who likes him, or finds him cute, tell him that. He might go and ask her out or else it will just raise his self-esteem.
If they try to kiss you, or anything of the sort, you are well within your rights to say no. Because that's not ok, not at all. Don't just go along with it, because you feel like you owe them at least that. Because you don't owe anybody anything, and they don't have the right to make you feel in debt to them for some reason. So just remember you can say no.
Do not be impolite,that is the purpose of this article!
If you are impolite, he will tell people and your reputation will be ruined. Also people won't want to go out with you.