Telkom deducting money from previous clients.

I had a 2 year contract with Telkom and after that I stopped my contract to go prepaid. After a few months Telkom suddenly deducted an amount from my account, so when I went to the bank to reverse and stop it, the teller told me there was another client that had the same problem. Telkom just deducts money if you were a previous client of theirs. And then they deducted an amount again today after 1 and a half years, and I still don't have a contract with them. How do I report them?