They came in on Thursday night, two days after making off with somewhere between three and six volumes of Battle Royale. They had one other girl with them who had not been with them the last time. And the employee who was there walked up, told them "We've been having trouble with stealing, and we think it was you. Get out and don't come back."
The one who hadn't been with them protested, but the others didn't say a word. They knew they were caught, and they knew that my employee had snapped photos of them on his cellphone so that if we have any further trouble, we can quite easily identify them for the cops.
Problem solved. It would have been more satisfying to catch them in the act, and when reordering volumes 1-11 of Battle Royale tonight (aside from 2 and 7, which are frustratingly unavailable at the moment) I gritted my teeth, but it's nice to have it dealt with so quickly.