Just tell them you're moving to another state/country. Works for me
That particular rep left me alone after that. I've received two phone calls since, to which I said, "I live in a yert in manitoba now" and the second call I just hung up. I still 9 months later receive occasion break up letters from comcast, asking me to come back.
It has already worked for me more than once (unfortunately, nobody ever tried to offer me free service).
If it even occured to them it might be sleezy, they'd rationalize that they do it to please their boss and keep their job, and that it's more important to them than being decent to a random stranger :/