Ex won't stop sending unappropriate texts?

My ex who is the father of my 2 boys, won't stop sending my unappropriate texts and emails. We have not been together for a year now, and he can't seem to move on. I've explained that I don't want to be with him, and the only contact I would like to have is about the children. He will switch from "loving me", by sending me sexually explicit texts of nude pics of him, asking to pay me for sex, or just sexual in nature. Too "hating me" by texts with severely hateful language. This has been going on for the past 6 months now, and is having a big impact on my life. Because I do have to have contact with him about the kids I can't just block him from my phone. Does anyone know if there is any police action or law that has to make him stop?