Messin with my bff's guy?

so theres this guy my best friend is practically obsessed with. and he told me that he used to like me. and i dont like him-like him. i just think hes cute and all. but he smacks my a .ss all the time and now we have this game where we go and smack each others back and forth. i know its childish but its flirting. and my friend always gets annoyed when we play around in front of her. adviceee??