A real customer service chat I had with logmein today:
This answer can be universally applied, any time you don't want to bother explaining to Joe Company, Inc. why you don't want to keep paying them.
Hey I get it that your explanation helps them tweak their products and services. But sometimes it's we end-users that lack the script for the appropriate response.
So if you want them to know, then by all means, tell them. But if they've already wasted enough of your time, or if, for any other reason you don't care to elaborate, say:
As you might have figured out, the idea is to avoid any depth.
And if, as happened to me today, they respond with some dire warnings about awful things that will happen to you if you quit them now (as in locusts, pestilence, or eternal lack of access) say "Could you just do what I asked you to do?"
There was no more delay after that.