I had been through this kind of situation once. The guy didn't really tell me he was into me, but I could tell from his questions. He would always ask if I had any boyfriend, my plans about the future, and several other such questions. I felt for him because he was such a nice guy with no known weaknesses, but I just didn't share his idea of the two of us together as a pair.
I talked to my mum about it, and he discussed it with her man, they told me it was better to say something like, "I enjoy every moment I spend in your company, since we are mere friends without any attempt to make an impression on each other or something like that". He was somehow startled to hear that from me, but it seemed he got the message that I was trying not to hurt him. He stopped talking to me after that day, which I didn't really like because I wanted us to still be good friends.
When I ran into him incidentally a few days after that, he acted like everything was cool and he had no hard feelings about the whole issue. I guess it would be nicer and easier to say something like what I said to him to any guy you want to discourage his advances without hurting him.