I don't call them a science denier - which is about the most self-contradictory term anyone could come up with. Science is the process of questioning and doubting prior science. You're not "denying science" to question science.

I call them "flat-earthers", as that's something that is usefully descriptive of their views and that they themselves seem happy to be called.

That is, it's a name that doesn't also necessarily insult them, which as I see it serves no useful purpose for anyone and simply causes antagonism and less of an ability to successfully communicate.

Which, by the way, is the only working solution to disagreement.