Hi Heather,
To answer your question, he's calling because he can. He's calling because you have allowed him back when he has done this to you and your children before. Why would this time be any different?
If you truly are "done" you need to "do" something about it. I may be wrong here but I don't think so, you cannot keep your children from him. You need to get some legal advise. Unfortunately those kids are as much his as yours. Protect yourself and those children, get yourself an attorney. Stay strong Heather.... Peace is in your near future!
Ro