I want them to stop sending me nuisance mail whether or not the contract is enforceable.

Did canceling stop it? They still have your contact information and they still know you have a satellite radio in your car.

I still received mail sent before I cancelled, I received a piece of mail acknowledging a cancellation and offering a new, discounted subscription. I believe I received at least one more piece of mail.

To be clear, I do not think any of my efforts will get my contact info out of their databases. Auto purchases are recorded publicly (at least in my state).

My comment above was about the extent to which Sirius, as a company, puts up hurdles to protect their nuisance practices, including shrouding them with legal claims that they will defend at the highest levels of jurisprudence. They lost their case in 2014 and updated the language in their agreement, presumably to address the weakness of their previous agreement, since it still claims to bind the customer without any action on their part.

In any case, I do not want to derail this thread any further.