No one can disprove the existence of any god or any other mythical character, even if the person who made him up admitted to making him up. There is still a possibility that he exists.
But any god could always prove his own existence. Just show himself to the entire world, even to people who previously denied his existence. Walk on water, walk through walls, turn water into wine, bring a person back from the dead, snap his fingers and cause the entire world to flood, snap his fingers to give a blind man back his sight.
I can see why some people are convinced there is a higher being who created the universe, I don't fault anyone for that belief. I just don't see why people believe in specific gods described in the Bible, Quran or other popular religions of our time.