The Blaze: Mob of Teens Reportedly Steals $3,000 Worth of ‘Nudie’ Jeans From Chicago Store
What I found interesting about this story was that the store owner knew something was happening when he saw the flashmob enter.
He locked the doors to stop the flow of thieves, but this is obviously not the right course of action, as locking someone in a place and preventing them from leaving is a form of kidnapping.
Only thing I can think is you hop on a counter and yell, "This store is now closed! You are all trespassing, leave now, or [insert appropriate threat for your locale here]."
Sadly, being in Chicago, the appropriate threat for that locale is, "I'll stamp my feet and gnash my teeth." Perhaps he could have yelled "Fire!" or something equally distressing, but there's probably a law against distressing thieves in Chicago.
Were this free America, he could hop on the counter with an empty shotgun, announce the store closure, and start corralling them out the door by walking through the aisles from back to front racking the empty shotgun (pointed at the ceiling, of course).
Seeing as the mob is borne of groupthink, maybe there's a trick to dispelling them. Perhaps whispering, "The guy's got a gun! Get out! Pass it on!" would cause the little lemmings to do as you say. Or the opposite route, walk around with handcuffs and single out each one, saying, "I don't care if I can't catch you all, I'm going to arrest YOU!"
If he knew they were going for those jeans in particular, he could have ran to the display, grabbed them all in bundles, and started tossing them behind the counter. Don't let the mob hide behind, "I was just wandering around looking at clothes, I wasn't providing cover for anyone stealing."
Either way, you need to do something that upsets their plan. You need to stop reacting, and do something to cause them to reevaluate their plan. Get inside their OODA loop. He did do something to disrupt their plan, but he didn't announce it to them to make them know their previous plan wasn't going to work. Because they're all groupthinkers, they wouldn't have the ability to adjust their plan without consulting twitter or whatever planning group they used, and would likely abort immediately.
Thinking about police crowd control, there should be a way to scare them with the threat of pain without actually delivering it. Maybe shouting that they're all trespassing, and saying you'll pepper spray anyone who is in the store in 5 seconds, then running around with a can of Fabreze painted black, spraying near people. I don't suppose you need a cloud of spray, maybe just the spraying sound will work. Just something to break the mob mentality. Paintball guns are a little much, and would leave evidence to clean up, but sweeping their feet with a scary looking airsoft gun would give them a visual indicator, an audible indicator (they probably don't know what a gun really sounds like, or have a fake suppressor on it), and give them impact on their legs (or would whizzing over their head be better?). If we want to get more overt, but definitely more effective, load some homemade blanks into a Big Scary Revolver™, don your hearing protection, and start shooting it into the air. Bet they scatter like cockroaches, and you have no BBs to sweep up, leaving no evidence of your method of crowd dispersal aside from the smell of cordite (Fabreze saves the day again!), though you might have to clean up some urine.
PS: You might have to lose the surveillance tape after some of these ideas.