Can a negative be disproven?

Yes. Science does this all the time.

Hypothesis; "Gravity is demons holding us down from soaring with the angels."
This hypothesis can be disproven by the complete lack of evidence in demons and angels, and the scientific evidence we have to show that gravity is not a supernatural phenomenon.

Why then do even some atheists believe that god cannot be disproven with the same method?
There is a complete lack of evidence for god, and undeniable scientific evidence to counter the claims of religious supernatural phenomena.

Thunder is not Thor and his hammer. Lightning is not Zeus casting his bolts from Olympus. These gods are accepted as disproven deities and mere mythology.

Why then is the Abrahamic god given special consideration and allowed to be considered above disproval... above mythology? Because it is a current religion and even the brightest and most reasonable people sometimes allow the social prevalence of religion to sway our reasoning minds.

After all, we're only human.