Boyfriend won’t stop

My boyfriend always touches my ass and spanks me and tries to pull my pants down and like dry hump me as a joke and won’t stop even if I tell him to stop and that I don’t like it. He thinks because we’re together that he can touch my ass or pull down my pants to touch my ass or do whatever he wants to me. He doesn’t take it serious when I tell him to stop and I’ve told him before that I really don’t like it but it never changes..