Mtn is frustrating me

Mtn is continuing to bill and create bank charges for for something that is çlosed. I had a contract with them that ended in November andy last installment was supposed to be in December 15th. But to my surprise I keep accumulating bank charges because mrn says I owe them. I gave them a call to cancel and the guy said they will do so by the 23rd of December. I don't know what to do anymore