So how would you definitely proof that there is no one who knows whether a god exists? OK, apart from the obvious option of killing everyone and then stating that since there's no one left, there's no one who could know anything – but then, in that case it won't matter any more anyway ;-)
Of course you'll also get into the problem of defining exactly what "knowing" means, and if you are not careful, you'll easily define it in a way that no human is able to know anything. Of course in that case, the statement that no human knows whether a god exists is trivially true, but just as trivially meaningless.
Indeed, in the case that an omnipotent god exists, that omnipotent god should have the power to let anyone whom he wants to know it know that he exists. Therefore if you could proof that nobody can know if a god exists, then you would, by this very act, have disproved god, and therefore could no longer logically be an agnostic, but an atheist.
But that means that to proof that nobody knows whether a god exists, you'd have to do the proof without denying the possibility that they could possibly know. However, since you can't know what happens inside someone else's mind, how could you exclude that they gained that knowledge somehow?