How to beat a player at his own game?

There's a guy who has a girlfriend but he is always paying attention to me and flirting with me, and he's even gone so far as to say "awww you didn't tell me you had a crush on me, how cute!" and I wrote back "key word being HAD lol" but I started thinking "You know, if he actually liked me he would break up with his girlfriend and ask me out properly." So now I want to show him I'm not that naive. But how?