At this point the person is cornered and not being reasonable.
You may choose to continue the dialogue, but I suggest ending it here by pasting...
I can see you aren't taking my questions seriously for whatever reason. I think it's best to end it here. Thanks for your time and God bless.
If the person finally admits he/she was being silly, paste this...
Okay, so I state again - You would have to have infinite knowledge (know all things) in order to KNOW for a FACT that God does NOT exist. Do you claim to know all things?
If the person asks where's the proof for God/Jesus/Christianity, go here.