If I was in this situation, this is what I would do:
Every time she exhibits the rearing behavior, drive her hard. Make sure to keep a safe distance from her flailing hooves, but drive her hard by making loud noises, coming at her aggressively (but not endangering yourself), waving your arms, shaking things, etc.
You may want to lead her on a long line if your going to be doing this, or else it will be almost impossible to do it safely. Keep driving her for five to ten minutes and do not let up. Keep her at a fast trot if not a canter (this also depends on the area you are in when she does it). If she acts like she wants to slow or stop be aggresive and drive her some more. Basically, it will show her that this particular behavior causes her to work hard and she will learn not to do it. Horses seek comfort, and workin hard is not comforting. Also, in a herd setting, dominant horses drive subordinates very aggressively when they do something that angers the horse who is higher up on the totem pole. You are establishing yourself as the boss.
If you belive in reading the body language of the horse, here are a few signs that the horse is giving in to your authority while you are driving (basically punishing in a natural horse way) it. The horse will flick its ears towards you, lick its lips, and lower its nose to the ground as it travels. It may do all of these things at once, or independently. The surest sign is the lowering of the head while being driven. When my horses show this sign of submission when I perform this exercise for a misdeed, I immediately quit being aggressive, turn my back to the horse and seem uninterested. I don't peek out of the corner of my eyes as this is suspicious and the horse knows. Most horses will slow or stop, turn and walk right in to you. If it doesn't, I'll resume my driving until it does this, but you don't have to take it as far as a huge training session on who's boss, though taking some time out to drive the horse and gain its respect is always a plus.
Simply driving it for a few minutes when it is bad will probably work as long as you do it aggressively and immediately. If you are in a place which you absolutely cannot drive the horse in a circle, then drive it aggressively back or to the side. Let the horse know you are serious.
I had a horse who reared over backwards when on the long line. She was a little insane and did weird things...I just kept at it and let her do it again and again until she decided it hurt and stopped.
When in the saddle on a rearing horse I've always had the best luck with smashing a water balloon between its ears. Most horses freeze up at the feel of liquid suddenly running down their head. I think they believe rearing caused them to injure themself and bleed and they stand there in shock. Most horses who aren't insane don't try many time after that.