I successfully canceled a newspaper subscription in like 2005, but for a few months, they'd call. The first couple times, I said I wasn't interested and hung up. Then I just stopped answering, but they still called. Then I finally answered once and said "Stop calling me" and they tried to say "If you want to be removed from our call list, you'll need to call our customer service line" and I said "No, that's not how this works. I asked you to stop calling me, so stop calling me." and hung up.

Surprisingly, they actually did stop calling.

SiriusXM never called me, but I got mail from them every damn week. I had even tried telling them to stop sending me mail, but still got it until I just changed my address to some bullshit fake address that didn't exist.