"Is there a mechanism for deposing a pope?"

Not that I'm aware of, unless of course it could be shown that the pope was elected fraudulently, as was the case with antipopes of the past.

The pope can resign at any time for any reason or no reason.

It's doubtful that Dawkins will be able to get within a mile of him. Also, he does a great disservice to his fellow atheists by portraying himself as a slimeball.