There are locks that allow people to leave through a door, but not enter through it. The store should invest in a few of those.
I'd call the fire marshal at this point.
I once had to deal with teaching in a university auditorium with entrances front and back. At mid-semester, the front entrances were chained shut. There were no locks on the outside doors, so they just chained them closed. I brought the matter up, and was told this was necessary for a weekend when an visiting orchestra would be playing in the auditorium, and needed to store their instruments in the lobby.
However, the chains were still there two weeks after the orchestra had left. When two more visits resulted in nothing and the chains were still there, I called the campus fire department. The chains were gone in two days.
That auditorium was designed to have people enter and exit from both the front and back. If a fire had started, or if a fire had started in the back of the auditorium, bad things would have happened.
The following year, I taught in the same auditorium and the same thing happened. But I knew to go straight to the fire department.