I think

they just need to ignore him.

Vatican plots battle with Dan Brown over Da Vinci Code sequel

While they claim that he's using the Church to make money (which he is) and lying about the authenticity of his information (which he does), by fighting with the fellow they're just playing into his hands. They took his entertaining, but ultimately forgettable, previous book and made it (and the following film) into a bestseller by fighting over it. They just need to say 'Ehhh, it's just him again. He hasn't a clue.' and forget about it...