First of all, it's not the 'standard' atheist position that "God" doesn’t exist, but that the case for the existence of such a thing hasn't met it's burden of proof. Likewise, it's not up to us to prove the non-existence of anything, even if that were possible. It's not reasonable to expect a person to come up with a definition of something in which they don't believe and then make the case for why it doesn't exist. You don't need to do that for anything else, from leprechauns to zombies, and that doesn't change just because the word "God" is plugged in there.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'