Continued invoice even after cancelling contract

I cancelled my telkom tablet contract on 12 July 2017. A week later I contacted the call centre and spoke to Sandy ********** She was excellent and asked me to send the cancellation documents AGAIN. I did that on 19 July 2017. Having done this (it's the ONLY way you can cancel your contract) I'm still being billed on a month-to-month basis. In the meantime, call centre agents are calling me with the following nonsense: Isn't telkom great? How about we upgrade your tablet contract? NOOOO, Telkom's NOT great! I don't WANT an upgrade. I just want to cancel my contract!!!!