Making her work harder is not going to do anything at all. She will not associate her "hard work" with trying to defend herself. All she will know is for no reason you are punishing her. And since you made the problem by allowing her to get bit, maybe you should be worked hard.
She is giving cues that she is going to do this, and you are missing them. Either take up on reins and tell her no, or make her speed up slightly.
But this is your fault for riding with people that let their horses do this to your horse, and for not putting a stop to it at that point.
For most of us? One time would have been enough. You have shown her you wouldn't defend her, so she has solved the problem, which has led to another problem.
I would imagine as much as anything your riding mechanics are at fault here, in that you are not keeping track of what others are doing, as well as what your horse is telling you.
Tie a red ribbon in her tail, and find decent people to ride with.
Horses make me a better person.