Is CELL C *****ing from me?

I am so angry and disappointed in the way my 2 years are ending with cell c. I got a contract in December 2015 with cell c, all was good. I received a job overseas and that meant that I would not renew my contract with them. I received a ton of messages about upgrading, which I wouldn't take as I was leaving. From my understanding our 2 year contract was over but NO. When they were sending the messages to upgrade my contract they did not state that I had to let them know I did not want the contract anymore. I went on the assumption that because I have not signed anything then we are not in contract. Cell C continues to charge me as if we still have a contract. When I called in I was told my contract could not be stopped until I paid what I owed in arrears. Obviously I owe cell c because I didn't know I still had anything to do with them. To me this is *****ing because I don't have a contract with them and they continue taking money from my account without my permission and now I have to pay for that. Had a good 2 years until all this.