Harresd by Telkom due to billing Incompetence

How do I stop harassment from Telkom for an account I do not have. I have proof of cancellation last year in July due to coverage. I handed them back their brand new cellphones. I have tried in every way to stop them from billing me and deducting money from my account, month after month. They simply do not respond or are too *********** to manage cancellations. What are they going to do next? Blacklist me?