I bought a new car that came with a 3 month free trial. The reception was horrible in the forested, hilly area where I live so I never used it. Near the end of the trial period, I started receiving calls from Sirius asking me to continue my subscription [the dealer had supplied them with my unlisted home phone number, which is another rant entirely].
I told Sirius I wasn't interested, but they kept calling me, up to 10 times per day, at all hours. I kept telling them I did NOT want their service and to please not call again. After about 2 weeks of this, they called again and I told them to never call me again. The caller said "Do we have your permission to call you?" I said "Weren't you listening to me? I just told you not to call me ever again." He went on to tell me that they have a specific script they have to follow, and the customer has to specifically say "You do not have my permission to call me" before they are allowed to remove my number from their call list.
Easier way is after the first time of telling them is to just file a complaint with the FCC. They'll very quickly stop calling you.