Just don't let her do it- try to keep her busy. I don't know what's your level of riding, but if you keep her busy even with doing lots of different sized circles, simple transitions (walk-trot, trot-canter), halts, etc, she'll probably stop having tantrums.
There might be other reasons for horses misbehaving, than boredom, though- wrong aids from the rider, badly suited saddle, which is rubbing the horse, etc. She might just not be used to having lots of horses around. Also, some mares can get really stupid when they're in season.