Telkom unauthorized debit orders

I had a contract with telkom which expired in January this year. But only cancelled it and blocked the sim card at the Menlyn branch in July. The telkom employee told me I will get the last debit order in July. But I recently found out they were still deducting my money. From August until November. What makes me more mad is that if they failed to get the debit order due to insufficient funds, they would try again and again. Which drove my bank balance to a negative. Then when I get money the bank will deduct the charges. I demand my money back.