DEBITING OF ACCOUNT WHEN ACCOUNT IS PAID UP

My husband took a contract with Mtn for a 2 year period and the contract expired this year, We called Mtn before contract can end to say that it's going to expire and we would like to cancel, they agreed but still debited the account when contract was over, we called them many times which they agreed that they will sort out. My husband as even gone to the store and they assured me that this has been now sorted and that he will not be debited,but yesterday my account gets debited again. i have to leave my work and keep going into the bank to reverse this and i get charged all the time for bank charges for their incompetence. We are now tired of Mtn, i guess that 's why people are afraid to take contracts. We are now tired of keep speaking to Mtn.