In my most recent case I received such a packet 6 days after the date they said they activated the service. I called the same day and told the agent I wanted to cancel my trial subscription, citing specifically that I did not want the service and refused the terms of the agreement. The retention script (which is the same no matter which agent you talk with) is, "well you can keep the trial going and it will just expire", and repeat it several times. You have to be persistent and use the language "cancel my subscription", or you will get nowhere.
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.