No, it is not generally legal to shoot an unarmed man fleeing a robbery.
Instead, smack him in the back of the head with the pistol if he tries to get up. Do it really, really hard. Put some effort into it. Make sure he gets the point.
Before that, however, he needs to really believe that you are going to shoot him if he looks at you or moves his hands from the position in which you put them. Be convincing. -MP