Wrong fully debited my account without permission

Ive been a client with mtn for over a decade and i have a been a cash paying client since i had signed up contracts with mtn. On the 29 of June 2015 i requested Mtn to debit a bank account assigned for mtn specifically on the 31 of July 2015 this was made after the account of june had been paid. on the 6th of July mtn Wrongfully debited my account even though my account was paid in full and i owed them nothing, there wasnt any money in the acc as the debit order was suppose to have gone off at the end of July and not the 6Th which then reflected on my credit at the bank as a fault i then called in again and cancelled the debit since the wrong debit messed up my record i was told that it has been changed to cash again and that mtn would not debit my account again. I then paid cash again end of July and after my payment has reflected they debited again which has now messed up my credit record at the bank totally. I called in and requested for letter from mtn stating that this was a fault on their side and all i got was we cannot authorize such a request&lt