What do I do when my ex-boyfriend is being totally immature?

Ok, so I dated this creep for a while (found out he was a creep after about 2 weeks into the relationship) and then I couldn't take it anymore so I broke up with him. I just didn't mean to do it on his 16th birthday. I feel bad since I was his first girlfriend and that's supposed to be one of the best days of his life, but he was being totally pushy and immature. Now that we're not together anymore, he keeps sending me texts about how stupid I am and in class he tells people things about me and yells stuff about me. He was really childish and forward during our relationship, and now he's really getting under my skin. Do I just completely ignore him or should I do something? If I should do something, what should I do? And is he acting like this because he just can't handle a breakup or is it because I ruined his birthday?