Just to provide a minor caveat here:
Whilst most the above replies (ie Dan's, mine and rumrunner's) appear to be
quite rude on paper, as with everything, it's all a matter of context. With
these type of lines, delivery really is everything. I would never think of
delivering such a line without a huge grin and a wee wink for the customer -
just to let them know that it's not a personal insult (although really it is)
and I will happily deal with anyone this whey. I imagine it's the same for the
others who have posted here.
Heck, I've told Johnny Depp to stop being a dick:
Me: "Johnny. Ewe're being a dick. Stop being a dick and buy everyone a drink".
Johnny: "Yeah, you're right" (he doesn't know about the ewe thang, ewe see)
and then he bought everyone a drink. It werks.