Vodacom does not want to cancel my contract

I had a 2 year data contract (wireless modem) with Vodacom. When expiring date came, I went to see Vodacom4U at Centurion Mall. A lady by die name of Thuli assisted me. I told her I cannot afford it anymore and want to cancel. She suggested a new contract that was on special that specific day. I agreed and we did all the necessary documentation. After a month I noticed that they deducted too much from my account. I said I cannot afford this and she said it was a mistake from her side. I asked her to cancel the contract. She agreed due to the fact that it was her fault. They refunded all my monies. She confirmed all was done, I no more has a contract with Vodacom. Now I still receive Invoices for payment of a contract that I did not know existed. I phoned Thuli and she said... oh she just cancelled the upgrade. After confirming I don't have a contract with Vodacom anymore. I owe more than R700 on a contract that I was not aware of and did not even make use of. They can confirm that. Now they don't want to cancel my contract because my account is in arrears. This is not a fault from my side but from Vodacom's side. I cannot pay for a service that I did not know existed and did not use. This is totally unacceptable. They keep on sending my issue to the back office with no feedback, just another Invoice.