Refuse to accept cancellation after expiry

On expiry of my contract, I informed customer service that I do not wish to extend the contract. They acknowledged receipt, gave me a reference number, and instructed me to also send the request on my company letterhead to the business section of Vodacom, which I did immediately. After 13 letters, they still bill be despite the fact that I do not use the sim (because it was cancelled) Bella, with unknown surname, who can not even communicate properly in English, are saying in a very unprofessional way, and I quote with her spelling errors, copy and paste \Our refernce numbers are like this one A1-X512-K9NU1and the one you are providing is not valid and that measn it nvere came to our box and our system is rejecting it