This is a common misconception about proper atheism, a total denial of god in existence or possibility. But I'll try to clear that up a bit. Anywhoo...

Atheism does not claim that god is definitively proven to not exist. Just as the existence of any being the mind can conjure up like the flying spagetti monster or invisible jellyfish, something non-existent can never be disproven since there is no supporting evidence capable of arising relating to something non-existent. Atheism merely states that there is no logical reason to believe in god, and until there actually is sufficient evidence pointing to it's existence, the belief in god has no more validity than any other imaginary being we can concieve of. Logically, you cannot say anything doesn't exist conclusively, since it may exist in some form somewhere in the universe.