Tell them they wasted your time, so you are wasting theirs.
I still get those damned duct cleaning guys calling. When they say "How are you today?" I say "I am going to be really annoyed if you are calling to clean my ducts".
The call centres calling are not in Canada. It is good to report, especially if you have the name of the company they are calling for. -- The irony of common sense, it is not that common. I cannot deny anything I did not say. A kitten dies every time someone uses "then" and "than" incorrectly. I mock people who give their children odd spelling of names.