They know where it’s going to take place.
Set up many cameras and record everything.
Seal the area.
Arrest them all.
Go through the camera evidence and charge them appropriately.
It’s racist to expect them to follow the law or to charge them for crimes more civilized folk would be charged with.
Advertising “bash mobs” is an eventual opportunity for vigilantism to occur. Removing a few from the gene pool would make the rest think a bit more.