UNABLE TO CANCEL TELKOM MOBILE CONTRACT

I am unable to cancel my Telkom Mobile contract. The contract has run out and I have submitted the cancellation forms and supporting documents in September 2017. After several calls to the call centre, usually with a response that "they can see my cancellation and enquiries on their system and will escalate it to their supervisors". It is now April 2018, and I am still receiving invoices and statements which is obviously escalating. I have called them again today (waiting 25 minutes for the call centre to answer) - sending all the forms again - again receiving a response that they will escalate it to their supervisors. What else can I do??????