I am not trying to stir up issues but I rather agree with Vaughan06. I think you really overreacted at first. However, I wasn't there and I don't know if at first you approached them nicely about it or immediately had an attitude. I'm one who usually holds my tongue at downright mean customers, but some people just can't handle immediately being attacked like that.
Perhaps if you nicely reminded them over the course of several weeks and had no good luck, then I would just go right to the manager.