I don't really understand why, if they are married, they are still living in separate states, unless it is a work thing. But whatever.
Can your ex-husband tell you a rational, logical reason WHY he insists on treating your daughter this way, and letting the stepmom get the girl's haircut? I mean, is it really about control with him? Why must he control that? Or are they finding her hair harder to take care of when it is longer? Or is this just some sick game on their part to piss you off, with your daughter as the pawn in their little schemes? IS he even allowed to take her across state lines?
Document everything - every time you've tried to contact the stepmom and she has not responded, conversations with your ex, etc. I agree that you should just write a letter to the both of them stating that under no certain terms are they to cut her hair without your consent. If they can't agree to this like adults, you will have no choice but to seek further legal action because your daughter's self-esteem and relationship with her father is being affected in a negative way. If she really needs a hair cut or asks for one, you will take care of it yourself. Your daughter can't be expected to be able to stand up for herself at her age - she needs you to advocate for her.
Personally, I would still be discussing this with an attorney in the meanwhile. I would be very reluctant to allow visitation if this is how her father is, and this is how they are spending their time, and the stepmother thinks she can just do whatever and not be bothered to communicate with you about it. I would also have a therapist on your side for you and your daughter to talk to, because she could end up with major psychological issues because of your ex's behavior and attitude. There's no reason for a child to have to go through what she is going through. They both sound like pricks.