Flat mates?

I always find it amazing that there appears to exist people who are apparently sincere about believing that our planet is flat. Goodness knows how they explain flying in a plane across the international date-line but in any case, there's so much obvious evidence (including photographic) for the Earth being a globe that it's almost impossible for any rational person to take such an opinion seriously. So, should we ignore, feel sad for or simply mock these people? Personally I think it depends on the context, if the person is obviously sincere and questioning, and not trying to proselytize then perhaps a softly, softly approach is best? Non-confrontational, respectful but firm and factual in a presentation of the evidence that refutes the position. On the other hand if the person is standing on a soap-box in Hyde park corner trying to flog their nonsense (or even tee-shirts!), then the gloves should be off! Humor, ridicule and satire are often the best ways to introduce doubt into people's eyes, perhaps not the zealots or the hucksters, but certainly the bystanders and the agnostics.