Ohiogal
Queen Bee
Actually her husband should state that in writing and her husband should do those things.The easiest and least costly way to do this is to lock the doors and do not give her a key. If she cannot get in, then you won't have her wandering around the house.
You could tell her in writing that any future entry she makes into the home with out invitation will be criminal trespass and that you will call the police to have her arrested for that should she do it again. The make sure you do call the police on her when she does it to back up your position. Whether the police will actually arrest her for it is impossible to predict. You can expect that sending the letter will likely increase conflict between her and your husband.
You could file in court for an injunction preventing her from entering the home uninvited. Again, though, expect that going this route will increase the conflict.
Note that since she has full custody she could simply keep the kids and not return them to your husband except for whatever visitation he was granted in the court order. If he has no visitation then she could cut him off from the kids entirely until he petitions for custody/visitation with the kids. That might well spark an expensive round of litigation to resolve.