Cancelled line that we still being billed for!

We cancelled a month to month contract in June 2018 and we still getting billed for a line that has NO activity on it. We went into the N1 branch to cancel, where we were told they cannot do it in store we have to do it online. Fair enough we went home and did it online and got the reference numbers etc. We still got billed and they then said no that will happen and once the cancellation is processed they will rectify the account. Months later nothing on the account has been rectified. We've been to the Telkom Canal Walk store who "Apparently" sorted it out but we still getting billed. My husband did a credit check in the week and his credit is bad because of Telkom bills??? Surely SOMETIME you guys will have to sort your customer service out! Please tell me HOW one cancels a contract without getting Telkom bills a year later? We have case numbers and references which doesnt seem to matter in this case. I need you guys to sort this out please?!