Telkom Mobile Contract impossible to cancel

Cancelled my contract with Telkom and paid arrears in ??May of last year. Kept getting invoices . Sent several emails plus phone calls and got promises that my query would be resolved and circulated. nothing happened . Till 2 weeks ago got email from National Debt connection. Shocked that i am even listed now. I have never met anyone as *********** as Telkom its like you are forced to stay with them .Its impossible to cancel your contract.