That's the problem with being "nice" in situations like this. Being polite is fine, avoiding cruel remarks and nasty behavior is good, but when you want to reject someone, BE CLEAR. Tell him in exact words.
"Humphrey, as I've gotten to know you I've seen that you are the kind of guy who can make a woman very happy. But I have come to realize that I am NOT that woman. I am moving on now. We won't be hanging out together.I see a great future ahead for you--but not with me! Bye, Humphrey, best of luck."
If you get an argument, listen for about 1 minute, then say: "I can see how strongly you feel about this, Humphrey, but nevertheless, it's over."
After that, don't continue any further conversation. If in person, and he continues, put up your hand in the STOP gesture and say, "Goodbye, Humphrey."
