Bad service from telkom

I lost my job last year and reported that they should cancel debit order since there is no money coming in on the account and that I will now pay the monthly payment of R100 by cash.to my surprise they still go to my account and I'm billed R202 every month for insufficient funds ...I now pay more than the agreed amount and they refuse to cancel the debit order causing me stress every month.I'm being told they can't cancel my bank details on the system