Oh, they tried the "relocation plan" on me.
Me: "Your service isn't available where I'm going and I don't have your phone service, so that really wouldn't help me at all."
Rep: "Oh. Okay."
If you do have the phone service:
Me: "Sorry, My new provider is wireless and disallows competing IP phone services on their network."
(You'll probably make the rep insanely jealous when you say that...)
Sometimes, after it's all said and done, they want a forwarding address to send the final bill.
Rep: "We offer service in that area, though."
Me: "Yeah, but the complex I'm moving into has an exclusivity agreement with [provider that's not them]."
Alternatively, if it's a single-family home:
Me: "I'm actually moving out the country and that's my grandmother's house. She doesn't need the internet for her typewriter or her rotary dial phone."
But by the time they get to the "forward address for the final bill" question comes up, you've already cancelled and they're just fishing for a new sale.