Most Helpful Guy
I once had a girl tell me at a crowded party, "Look you're a really great guy, but I can't mingle now. There's lots of girls here who'd like to chat you up around here, so go on out there and find her." That was the kindest rejection I ever had.
The "I have a boyfriend" line works too, just as long as you don't sound like you're lying or being a bitch about it.