I'm being blacklisted for their incompetence

I took a router last year (2016) at gateway shopping centre telkom in September. They did not check if there is coverage where I'm staying, they had my physical address. I returned it within 7 days because of coverage issues. They told me the case was logged I can keep it until I get a call from their technician. The technician called and told me to take back the device because there is nothing they can do about my coverage problem. I took it back last year. Telkom kept on taking my money every month. I ended up blocking the debit order through my banker. I kept calling, asking for my refund. I got a number of references but not my money. Today I got an email from their attorneys saying I have to pay the money that is outstanding. They are blacklisting me for something they could have resolved last year. What a bunch of losers.