OK, we don't have lightning fields or house shields like House Atreides or even personal shields.
My first thought, like many here, was "sprinkler system releasing pepper spray or tear gas, or just open up on 'em with a 'riot extinguisher' like they use in prisons." And I loved the flashbang idea. But let's face reality here:
If you so much as lock the doors so people can't exit (instant violation of fire safety code), let alone hose 'em with pepper spray, YOU will be accused of escalating things. Some of those dirtbags are bound to know bottom-feeding lawyers. There will be people who were gassed or flashbanged and later testify "I wasn't stealing a thing, I was just there and the storekeeper attacked me!" Life for you won't go well. As a 7-11 clerk, the only thing you know for sure is that Southland Corp. will throw you under the bus. Those punks all know that nobody will touch them, for just those reasons. They are counting on it.
Frankly, I think the ability to quickly step through a back door that seals well, and hit a button to fill the room with CN (not CS) gas, would really be nice. CN is easy to air out afterward, and it's adequate to transform a room into a very uncomfortable environment. But all that preparation costs money.
It's my understanding that in really, really bad neighborhoods, not only fast food establishments but service stations and convenience stores, are already set up with both the staff and the merchandise behind armor. The customer comes to the bulletproof window and places his order.
This kind of behavior defeats the "supermarket" concept that evolved in America. The obvious answer is to go back to the old time store, where the customer tells the storekeeper what he wants, maybe even hands him a list, then waits for the storekeeper to assemble the merchandise.
Yes, I know, if I run a store after TSHTF and TEOTWAWKI, things will be different. But we're talking about solutions that will work in today's political/legal environment.