No Response to numerous emails

I had a contract with Vodacom that expired in December of 2016. Due to the fact that I wanted them to convert the contract to pre-paid. I phoned them during December and asked them to cancel the contract and \I was given a cancellation reference number. In March of 2017 I received a call from Vodacom that I owned Vodacom money and that I would be handed over if this was not paid. Again i told them that I had cancelled my contract and the operator told me not to worry and gave me another cancellation number. Its now September and since then I have received numerous calls from MBD Collection Services on behalf of Vodacom querying the same account that was cancelled months ago. No matter how many times I email info through to this firm , nothing seems to come of it and the next month I receive another phone call. At the end of the day due to a mistake made by Vodacom I could be blacklisted. The lack of feedback by MBD services is a major concern. \how many times do I HAVE TO MAIL YOU AN AFFIDAVIT before you respond???? Regards Allen