My experience was similar. I think I blocked the phone number they called from, and that seems to have been effective.

Unfortunately, though not a subscriber, my car stereo often defaults to the Sirius input when I start it. This mean I'm greeted with Sirius ads when I start my car. Usually, this ad literally consists of playing a really annoying buzzing/screeching sound, followed by someone explaining that if you subscribe, you won't have to listen to that sound. That's an .. interesting .. strategy.