[VadeRetro]

That position is not necessary to be an atheist and I'm sure there aren't many who hold this extreme view.

I don't know how extreme it is, but one militant athiest told me he could prove there was no God. I was very interested, but whatever it was seem to dissolve in the telling and he never mentioned it again.

[BMCDA]

And I'd be interested too ;)
However, depending on how this deity is defined it can be quite difficult to disprove it. A god like the Greek Apollo is rather narrowly defined and so easily disprovable (but only to that extent that he doesn't exist as defined; he could very well exist but doing something else than moving the sun across the sky). The monotheistic deities on the other hand are more complex and thus harder to "disprove" in the above sense. Nonetheless one can point out inconsistencies in their definition.

[Quila]

but one militant athiest told me he could prove there was no God.

The militant folks on both sides tend to be almost comically unable to make a good case, don't they? I've had people try to prove both God and No God to me, and they never made much sense, usually getting into a really nasty tangle of logic that they strangle themselves with.