Spam, spam, spam, it never ends

I have now asked (instructed, if you would) Cell C numerous times to remove my number from their marketing list. Every time I (supposedly) get "confirmation" that I will no longer be spammed by unprofessional agents, yet time and time the irritation flares up after a month or two. What does it take for Cell C to actually listen to their subscribers? And more so, to leave them in peace?