Sirius is insanely persistent in calling - be prepared.  You can tell them "No" and the calls will keep coming.  You might call the car dealer, and the sales person you dealt with and let them know you will NOT refer anyone to dealer and the sales rep unless they make a good faith effort to get the calls stopped.  They will try it tell you some BS about how it is all the fault of Sirius, but say just they want to be in bed with Sirius, doesn't mean you want to be invited too.

You want to make clear that when people ask about the new car, you will make sure to say "I liked the car but the sales rep and the dealer stink".  You have little clout with Sirius, but if dealer start losing sales, they will start pushing Sirius at some point.