Have you tried just sitting her down for a serious talk about it and explaining why it upsets you so much that she does this? In the heat of the moment sometimes it's hard for the other party to see exactly how they're hurting you, she could be too focused on the hair itself and not on you, so sitting her down for a talk to focus on the feelings rather than the hair might help.

If you've already tried that I'm afraid I'm out of ideas. Some people are just like that and can't be changed... But I hope she will listen to reason.