Most of the sharper atheists I know don't "believe in" anything. There are things we have relatively strong certainty about and things we are unsure about. The things with strong certainty are things we are said to "know" and the things we're unsure about are things we are happy to admit we "don't know."
When things are imagined, we view them as not literally existing. When people imagine a thing and then pose the hypothesis that that thing exists in the tangible world it is not incumbent upon others to either believe or prove the thing doesn't exist. Nor is it inappropriate for others to assert, "No, that thing doesn't exist."
This doesn't pose a problem or a conflict because we allow things we "don't know" and have previously asserted don't exist to become things we "know" are real when evidence comes along to build certainty in it. As for god/gods/God/ghosts/souls/sasquatch/chupacabra/sharknados/etc. we're simply waiting for evidence and in the meantime we can appropriately say, "No, those things do not exist."
