What should I or my husband do about his kids Mother! She is driving me and him CRAZY!?

My husband and I have been married for 8 years and have a 5 year old together and for the entire 8 years his kids(2 daughters) from a previous relationship (non marital). He pays childsupport faithfully and she hates the fact that he loves his kids very much so she uses them against him every chance she gets. She tells him he can never see them again unless he does whatever it is she wants him to do at the moment. When its convenient for her (she needs to get rid of them) she tells the kids they can come over. She has gone as far as calling the police and gives false claims against him to get him in trouble...She works in dept of corrections and uses her job as a threat against him....She has his family under her thumb they believe anything she says and always let her off the hook.What can we do about her? He says he is sick of her and wants to tell her to keep the kids home PERIOD because it is so stressful dealing with her but he says he has no support from his family and they will continue to allow her to bring the kids over and then call him to come and get them and he is tired of it! What would you do? Please dont say take her to court because that doesn't seem like an option because she has all these connections and she is very vindictive.