How can I get my ex to stop harassing me when I can't use a restraining order?

I have been divorced for over a year and my ex still flip flops between trying to get me back and being harassing. He contacts (or attempts to contact) my boyfriends, texts me all the time (at least 10 times per day). We have two kids we share custody of 50/50 so there HAS to be some communication, but this is really insane~! He says he needs to "inform my boyfriend" of what a piece of **** I am. I know I can change my cell # , but there are times I have to use it regarding our kids... I am at a loss... any suggestions?