How do you deal with someone who thinks the Earth is flat?

Imagine you are relaxing at a bar enjoying a drink after a hard day’s work. The person next to you strikes up a conversation. Initially he seems reasonable. But soon he begins to go on at length about how the Earth is flat and how a misguided cabal of scientists hides this truth from us. You could try and persuade him of the error of his ways. But the most sensible course of action is to make your excuses and leave.