I've just cancelled my DSL after years of reliable service, for which I was happy to pay. You see, I've moved, and the package deal for the cable modem at the new place was much better.
Having cancelled my service, and paying what I thought was my final bill, I got billed for an "early cancellation" fee a month later. An early cancellation fee on a land-line DSL.
Now, this is total bull. Oh, I've paid, only because otherwise you will attempt to destroy my credit rating, but if your service is so crappy that you must threaten to charge customers for actually NOT using you, then you've got a problem somewhere.
So, my idea is, drop the cancellation fee. Simple good will.