Vodacom dont cancel contract although cancelled by me in time

I phoned to cancel my data contract for 5gigs at R199 per month initially in September 2018 when I was due for upgrade. I got a call from Vodacom to say I must call 30days before the end of contract to cancel. I again called on the 30 October to say please cancel the contract as from the 30 November/1st December. Confirmed by Lita reference number ********** 42. Vodacom has now charged me 259.00 for the 5 gigs for December. I did not use the data as I have another data contract. I spoke to Veronica at cancellations and Kaya at accounts today and they both said the contract will only be cancelled at the end of December because I am calling in the middle of the month. I just believe that this is a ******* of R259.00. Both cannot explain why they charging me the full price of R259.00 whereas my initial contract was for R199.00