Unwanted Data

I got a phone call from them end of November, trying to sell data.. As I explained to the lady, I have a data plan with Mtn and Fibre at home.. I DONT NEED MORE DATA. No mam, you wont be charged extra.. She said the amount I pay MTN every month will stay the same. And I asked her numerous times if she is sure. So, now I am getting this data I did not want and being charged for it.. I 'cancelled' it on the 19th of December, still got billed for it end of December and still waiting for my sms from them confirming that it has been canceled. I did not take the lady's name that phoned me.. But I am sure they can listen to our recorded conversation. I dont want this data!!!!