Is your horse boarded inside? How often does your horse get out in the week and for how long? Normally this happens because of boredom. Or it's a behavior issue. If she gets turned out with other horses, then she may have picked up on the biting from being out with them.
What I would do to stop this is probably instead of her going out with friends have her be worked for about an hour. Or have her be alone for a bit in the pasture.
This is all how you board your horse. If she is already outside with other horses then your horse just needs to learn respect when it comes to people!