I used to work with a guy who would look you straight in the eye and say, "That's very interesting... Excuse me!" He said it so quickly you knew it was a slight... But he was nice about it.
It's always nicer when you can tell someone to go to hell in such a way so that they look forward to the trip!
Get religious - arms outstretched, booming, old testament voice, "GO YE FORTH AND MULTIPLY!"