Next time just grab something unhealthy from their cart, look them in the eyes and smile saying, "Hey, you don't need this!" Then replace it with a carton/bag of lettuce and skip away with the item.  Several times folks here have shared tumblrina "thatHappened" sob stories where fats claimed that such things happened to them...it would be quite fantastic if someone did it for real.    An idea:  grab the carton of ice cream/doughnuts/corn dogs from fatty's cart.  Say loudly, "Oh, hey!  Thanks for reminding me!  I was supposed to pick some of these up for my sick anorexic roommate who just got out of the hospital--since you don't need them, I'll just take them off your hands!"  Then just drop the beetus in a freezer on the way to the check out.