What do I do!!

I've been with my boyfriend for almost two years. There is this guy living with my dad who I thought id be friends with(to keep it civil when I stay at my dads) and he became obsessed with me. Like constantly texting me and if I didnt answer he would text my little brother and ask him what I was doing. He is telling everyone we kissed and cuddled and shit. But it never happened!! I don't know how to get him to stop and I don't want my boyfriend to question whether I did it or not. As of right now he believes me but how can I get it to stop?!?!