I agree with most of the posts here that the LW knew perfectly well that he obvious solution is just to ask nicely that the person step back. I would (and do) ask people to step back if necessary, but I always ask, not order, and say please. I wouldn't just say "You need to step back" or give a cold stare. Sure, the people crowding me are wrong, but as m2kbug pointed out, they probably don't realize that they are too close or just aren't thinking about it. Anyway, being polite about it works just as well as acting as if you presume they are identity thieves.