How to stop my husbands ex wife's slander?

My husbands ex wife has been making awful statements about him on a social network. They have been apart for 3 years and divorced for 2 and she just can't leave him alone. They have two children together, so there is no way to simply ignore her. She makes statements such as he is a deadbeat father who never wants his kids around, and lately has been making harassing statements about our unborn child and our relationship. She says things such as he will walk out on us and go impregnate a new wife, and that my child will be all messed up because we are his parents. I have her blocked simply so I don't have to look at any messages from her, however she puts this stuff everywhere on different peoples pages so everyone in the world looks at it. Today she did the post that the father is a deadbeat thing because he won't take his kids, while we were in a meeting with his army recruiter trying to better his life and career for them and us. It's not even one of his days, she just wants to go out and can't afford a sitter. This is getting painful, and annoying. Picking on my unborn son has crossed my line. We are getting phone calls from friends about these statements she is making. I can't talk with her, and neither can my husband because she is so out of control and unreasonable, she just posts more afterwards. Oh, also she never uses our names, she hints at who we are ( everyone knows what she means) so I don't think I can get her with slander or defamation of character. Does anyone know if this can be stopped or how???