Arm and train the clerks. If the police will not punish then the staff must defend or the business will not be able to continue.
Shoot & kill the rioters and looters; it’ll stop real fast.
The possibility of death, to the average person, is deterrent. Not to most of these kids.
Beside that they will, in all probability, have guns of their own and start blazing away. If they "lose" round one with you, they will regroup and hit you again, this time with more gusto.
I agree with your statement but for these situations (flash mobs) I don't think it practical.