How can you prove there is no god if theist ask for it?

It's a matter of what and how you believe it.

Why would you want to prove that God exists? There are more important things. Do you want to say "Ha! I am wrong and you are right?" I will say this though, to anyone that says there is no God. Where did it all come from? A big bang? Possible. Would be foolish of me to say it didn't. I wasn't there. But that is only a guess, as you too were not there. But if it was a big bang , then where did that big bang come from? A ball of gas? Where did that ball of gas come from? The belly button lint of a flea? Where did it come from? You would have us to believe that these things came out of thin air for no reason at all, and no purpose at all? I say atheist have more faith in the impossible than believers. If you want to know how to increase your faith observe an atheist. They believe without signs or miracles.