Horrible service

I had a contract for laptop with MTN so the last payment was on the 26 September 2023, they debited me more than my monthly payment, I went to the glen mall branch in June to cancel it so that they don’t debit me more this month but they couldn’t help they say I must call on the 11th of September to cancel, I called and the guy at retentions said I can only cancel after the last debit, to my surprise I was debited more on the last debit