Select the appropriate setting. It is unlikely the young man would be pleased if you decide to announce that you don't want to meet him, at a noisy party where you just came from. Also, do not tell him, being someone of you in the apartment. This is best done in a neutral place - during a walk in a quiet Park or sitting at a secluded table in a cafe.
Talk to the guy. Calmly explain to him that he is kind, cute and really like you as a friend, but the passionate feelings you have. You should not tell the young man that he is a brunette and you like blondes. Emphasize his dignity and say that he'll meet a girl who will appreciate it. Most likely, the boy is still upset, but no reason to hold any resentment toward you he will not.
Can't decide on an honest heart-to-heart – write a letter. No-one will interrupt, and you will be able to point to Express what I think, and to dot the "i".
If a young man just trying to hit on you, tell him you have a husband or loved one, and tear him you do not intend to. Male solidarity still exists, therefore, most likely, the hapless gentleman will apologize and leave you.
Between you and there was not anything serious, but you think that guy is prepared to take decisive action? Gently tell him that you have a boyfriend. To the question "how are you", choking with delight, tell us what went on a date with your favorite Beau. Such news will immediately cool the ardor of the hapless lover and it does not undermine his self-esteem.
Once the conversation tell the man who stuffed you in the cavalry, which recently split with the love of your life and now you want to live freely. Can also, making a sinister expression to tell that will never build relationships with men. If you speak convincingly, the young man would believe and go look for the lady in another place.