"Bewahren Sie bitte die Etiquette!"
With a nice (was heißt 'verbindlich?') smile: "Ich erinnere mich gar nicht Ihnen das Du angeboten zu haben".
Depends much on the tone, whether it comes along polite. If you lower your volume, so that others around you don't hear it, it is really polite, since it doesn't disclose the dumbness of the other persons. On the other hand, lowering your volume but not enough gives the impression that you tried, and makes the other look very stupid (which I enjoy the most). Then, looking around with surprise, and correcting yourself, offering the 'DU': "Wenn Sie wünschen können wir natürlich gerne zum Du wechseln!" lets you look diplomatic, while you aren't.
"Wenn du keine Manieren hast, dann hast du keine Manieren!";) – Hauser Sep 7 '11 at 10:48