This may vary from state to state, but I had a similar situation. As my attorney explained it to me, it is very difficult to get a judge to order someone not to say something in advance. Basically, this amounts to a gag order and you need a pretty strong reason.
She said I had 3 options:
1. Wait until she said something - and then sue her for slander. Unfortunately, you have to be able to prove that she said it AND that you were harmed.
2. Ignore it.
3. Get something written into the child custody order stating that both parents agree that denigrating the other parent in any way is harmful to the child and will be considered contempt of court (sounds like it may be too late for that in your case).
I guess there's also one more, but I never discussed it with the attorney:
4. If it continues with enough frequency and severity (AND you can prove it), you could make the argument after a while that it's harmful enough to the child to justify a change in custody. I suspect it would have to be pretty severe for this, though.
While I got #3, much of the damage had already been done, so I am currently just ignoring it. For a while, there were a couple of parents who wouldn't let their kids come over to play, but they got over that when they saw that my daughter was with me every other week with no objections or problems.