The only way to defend against a Fat Damon is to address everyone in the vicinity, and in the most retarded voice possible proclaim, "Faaaaat Daaaamon".
Guy2: Approaches the bar gaining the attention of all around and proclaims in a retarded voice, "Faaaaaat Daaaaamon."
Carrie: I do not!
Doug: Oh yeah? What about the time you called me Fat Damon? That was five minutes ago!!