I would not even call their beliefs stupid since it sounds too much like calling them stupid and I would rather not open myself to them hearing that in what I say. There are perfectly devastating, highly specific and unbelittling words to use. You can say their positions are unsupported by evidence, fantastically implausible, absurd, fallacious, historically disproven, scientifically impossible/disproven/unlikely, logically contradictory, etc.
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