The problem is by no mean saying, "I don't believe in Satan" but saying, " I know for a fact that there is no Satan."
And we know for a fact that they cannot know what they claim to know for a fact. We understand the limits of knowledge on the matter even if they don't, whether they claim, "I know for a fact that there is no Satan" or the opposite one, right? It's like one saying that he knows for a fact that he will live beyond an age that he hasn't reached yet. No he doesn't.
And so we ignore such claims, or translate them into what they really mean - a person has taken a leap of faith and is confusing his own certitude with fact.