Tell off nicely a way older guy

So I am 18 and there is this 30 year old that just wont stup subtle hitting on me. We met on a gamesconvention with some friends of mine, and I believe that a good friendship can be, no matter the age. We share the same kind of humor and the convention was great, we made a group with him (my friends and me), means he got my number. Thats a while ago now and we are chatting sometimes, not to often. But he's always a bit too nice. A bit too agreeing, if you get what I mean. He's a nice guy all in all, but I really, REALLY am not interessted in him in any way. In fact, I even have a crush on a girl ATM. But I don't want to be to rude to him, since he didn't really make a move, or be gross, or really flirt. But that subtile shit doesn't get unoticed. How do I tell him off nicely?