bought out contract but still being debited

I cancelled my contract 2 months earlier and I paid the required amount asked of me in March 2015. I was aware I am still liable for another contract I have which is R200 only. The end of April I was debited the normal full amount - when I call the call centre I got through to 10 different people who all transfer me from pillar to post. I had to do a stop order to get my money back. I received an sms on the active contract I have a credit of R419 to my account - I calle them and they told me because ibought the contract out early. I confirmed I owe them nothing. The very next day I received another sms I owe R820 which is the old and still existing contract. I called again and go the same call centre problems. I should only owe R200 for existing contract April and R200 for May 2015. Vodacom is the worse service provider, their customer service sucks! I refuse to pay anything until this is resolved. If it is not resolved I will take this matter further