Oh, I hate people that do this. I have gotten a few cat butt faces by turning completely around and standing in line while facing them, then moving up as the line moves. For some reason, they don't do it when you're facing them.
I just try to stick a cart between me and people like that. Since I seldom go shopping alone, if I can not get the cart there, my Guy will turn himself into a human shield so people don't get too close.