Vodacom - double billing after contract cancelled

Due to me moving abroad, I requested early cancellation on my contract and paid the relevant quote in February, and opted to change to prepaid (keep my number) Needless to say Vodacom hasn't stopped billing me on a monthly basis!!! After numerous emails, eventually got a reply that they will give me a credit note... this has still not reflected in my account! And they actually had the nerve to deduct another debit order. How can such a big company act actually ********** and double deducting for billing after contract was cancelled and amount paid??? The lack of customer service and response is also shockingly poor; and I have been a client for more than 6 year.