I think my answer (after being asked so many times) would be:

"I've had horrible customer support from a Comcast rep, and I don't want to continue to patronize a company that would hire someone who acts that way."

(The next logical step is to ask for the offending rep's name, to which he could respond "what was your name again?")

What's the retention angle from that point? Is it "I promise if you stay with us, I'll go right to my boss and have him fire me"?