Take your time and think about it to be sure that you really dont want to go to prom with that guy, try to talk to him privately, and tell him that he is a nice guy but you already have a plan for that night (something else to do or you already have a date for the prom...)

ask sum1 else in front of him and pretend as though you dont know that he is there... be all weepy n stuff about not wanting to hurt his feelins n everythin... mk sure he hears everythin...