Whatever you do DO NOT GET PHYSICAL with anybody. I have been involved in a couple of lawsuits and spent a day in jail for trying to protect my equipment from those type of people. It has cost me many thousands in lawyers fees-settlement cost etc (not to mention the equipment that was torn up), and got me just inches away from divorce. If you have an issue, pull the plug (done that several times) and get away. If anybody goes nuts-then you can sue them! I lost 1 case on the basis that I was simply bigger than the person who brought on the suite. The courts are simply not interested in justice. They just want to get home to dinner and do not want to look at any evidence or listen to witnesses.
Simply walk away and let the proper security handle it. Believe me that is VERY hard to watch the situation, but the judge will tell you that security is NOT your concern-you are simply there to provide sound. Security is somebody elses responsibility. IF there is no formal security, then the problem rests on the person who put on the show.
This is a very ugly business, and it gets dirty at times.