A boy keeps doin things to my bf?

This boy called mitch tryed kiss my boyfriend! and he tryed goin down his pants aswell ! He is also in my bf's band , And this mitch added me on bebo, I want to say something to stop him doing things and i dont know what to say or do, He also says to my bf why does he like me so much and stuff like that, And my bf says he thinks he is a bit bi, I dont mind that but my bf keeps telling me its pissing him of and he doesn't know what to do either. Please help!!