How can you prove there isn't a God?

I can't, but I can reasonably conclude there isn't. However, I don't have to prove a negative. For instance, I don't have to prove unicorns don't exist to you, do I? As reasonable people, we can conclude that since there's no good evidence for their existence, they probably don't. If somebody insists they do exist, I'd expect them to know they need to show me good enough evidence before I'll believe it. Likewise, if anybody comes up with good enough evidence for me to believe in supernatural beings or the Christian God of the Bible, I'll believe.