Continuous debit even after cancellation

In November 2015 we emailed retentions department to cancel our 3G contract which was expiring in that month. We received confirmation that the account was cancelled. Our contract was a 1 gig fixed per month contract to the value of R105 a month. On the 1st of December we were debited an amount of just over R2000. We contacted MTN via telephone and email and no one can give us a valid explanation as to why. To our surprise on the 1st of January we were debited once again with an amount of R4200. Can someone please explain the logic of this when the account has been cancelled. MTN we will keep reversing the debit order until someone in your accounts department can actually reconcile an account. It seems that no one there knows what is going on and you are **********ly debiting people accounts without a valid contract. We are not the first person to have experienced the same thing. We will not pay this, we will keep processing a stop order until you reverse this ********** amount. Strange how we were debited correctly every month to the value of R105 monthly ( again read my words- FIXED amount monthly) until we cancelled the account. To top this off getting hold of MTN is impossible