Not a perfect solution, but here's what I suggest: Place a placard at the entrance to the restaurant that clearly and unequivocally states the following:
1. Entrance to the establishment is by reservation only and/or with the express consent of the management or its designee, and management reserves the right to prosecute to the fullest extent of the law anyone who enters the premises without such reservation or consent or remains in the premises when asked by management to leave.
2. For the protection and safety of all our customers, this facility under video surveillance.
All of this, however, is totally worthless unless the business prosecutes. If it doesn't, then the business loses the presumption of innocent in the court of public perception.
The targets of these flash attacks also need to take personal responsibility and stand up to these thugs. I don't mean a physical response, but hold the ground and refuse move until LEO arrives. These thugs are not going to physically harm anyone with witnesses and camera anywhere. Stay seated, call 911, and hold your ground.
“here’s what I suggest: Place a placard at the entrance”
They violated the verbal commands, why not a placard?