do what i do when i have a gf(very rare lol) if a guy is chatting her up just walk over whilst he is talking to her...put your arm around her and kiss her on the cheek. then look at him smile politely and say hi.
she will be impressed as you havent flown off the handle...he will not know how to take you so will stay clear. i love doing this!
almost makes having a gf worthwhile! haha