Telkom doesn't want to cancel my contract

I had a contract with Telkom mobile.I cancelled it in September when it expiered in The Grove branch pretoria.Their debit order keeps on going off around the 9th and it cause the bank to charge me because after the 5th sometimes there's no money in my account.I sent cancellations department e mail to cancell they haven't came back to me and again today they debit my account ********ly.Cancell this contract please and stop debiting my account.