Threatened with blacklisting for closed account

I am now being threatenedd with blacklisting and legal action via NuDebt, but I have explained to the NuDebt consultant and to a Telkom consultant that it is a Telkom error. This phone line was cancelled in March 2017 – we no longer live in Durban and don’t use this facility! I sent this cancellation form several times last year AND spoke to a Telkom representative to confirm it. What else can I do?? I refuse to pay an account for a service that I cancelled and no longer use!