I don't remember where I read it, but apparently one person was successful with stopping a free weekly paper from being delivered by reporting littering to the police (they tried telling the newspaper first, but they didn't stop).
I just say, "Cancel my subscription." Don't play their game, don't give them an inch, if they ask why, just repeat "Cancel my subscription". If they say something else, just say "cancel" and nothing else. They will cave.
I bought a new vehicle that came with a free year of SiriusXM, but I never activated it because I've heard how scummy the company is about canceling.
Once a month they mailed me a postcard to sign up, I called them to ask them to stop mailing me a post card, after a few back-and-forths, the person said, okay, we won't mail you anything for two years. I said don't mail me ever, I will never ever use them. The person said okay, they've taken me off their list. We shall see...