Proving that god is created by man is not quite the same as disproving his existence.
Well, technically you cannot prove anything does not exist.
What you could prove is that it spontaneously appears in human culture, without an apparent catalyst.
Come to think of it, if no religion arises, it might prove that God does exist because in the absence of an specific action, like the appearance of a Prophet that could be attributed to a God, it means a God must take action in a Godless society to become known.
Premise: Humans are naturally atheist. The idea of a God must either be externally imposed or God must take action to create belief.
Test: In absence of an external source, do humans start to believe in a god?
If they do not start believing in a God, that means there was an external force that created the belief in God in the past, therefore God does exist, and has not yet reveled themselves to the humans on the planet.
If they do start believing, it could either be: An act of God. Or, a spontanious creation of a god to worship. Either way, we cannot prove how it came about, so we have no proof if God exists or not.