still charging after cancellled contract

I cancelled my contract with mtn in january 2016 as i was unhappy with their service. On the 27th of january i made a cash deposit into mtns accounts to the value of R752.74 as a settlement value provided by the cancellation department. After which i called the call centre to cancel and port my number. In febuary i moved to another network. In march my account was debited with an amount of R2855. I reversed the transaction and went the mall of the south branch to query the amount. My contract was for R 200 a month and therefore the amount debited is 14 months of my subscription. The call centre was contacted and a reference number *** was provided. They could not solve my query as my details did not reflect on their system and i was told they will call me. To date i have not received feedback. Since then i received 3 messages from mtn insisting i pay the amount or they will take legal action.