Peaseblossom
I think that shop assistants should be allowed to use pepper spray if they know that someone is taking something, and that will give them enough time for the police to come and sort them out. Although they'll probably just get a slap on the wrist anyway. If there's a sign on the door warning that this will happen, maybe it will stop people.
Pepper spray is illegal here but if you happened to have a can of Deep Heat about your person, that might just work instead..
