Well, if this person is speaking to me, I move on in that I just don't bother with them. They can flap their jaws as they like, but I would just carry on with the discussion if involves others or just ignore them if it doesn't. They are happy to carry on their crusade elsewhere and I've no obligation or interest in changing their mind, assuming that is possible.
Could be very very fun, to wind people up though. Is a bit harder with race as you need to avoid sounding racist in the attempt. Is much more fun with atheists.