Cancellation of contract

My contract with Telkom which consist of a laptop with 500MB(which I never used) expired May 2017, I send them ALL documents to cancel contract, however they never cancelled the 500MB deal. I resend them the documents for cancellation and contact them, still no answer. They ended up debiting the amount from my bank account, luckily I realized soon to ask bank to reverse debit order. It's almost a year after I cancelled and I refuse to pay the outstanding amount that they keep on sending me. I will never take another deal from them.