Fire em!
Then take out a big ad in the newspaper explaining that the IAM goons assaulted 5 guys and that US does not condone this kind of violence and thuggery. Then explain that everyone else threatened to leave because the goos were fired.
Apologize to passengers in the ad, but state that US is not going to kow-tow to thugs and goons, and wants to provide the best, safest environment to its workers and passengers, and that the next few days may be a bit rough, but that US is building a more ethical airline with quality employees.
Passengers will take the company side, and the unions will rightly look like the no neck creeps that they are.