horrible service

Settled and Closed my account in February. RCS sends me an invoice in March. I phone call centre. They can't find my account and assures me the it is closed. Then I get an sms a week later threatening me of legal actions if I don't settle outstanding amount. I go into the store. They can't assist. I need to call call centre again. Call call centre just to spend 15 minutes listening to a computer and entering numbers. When it comes to an operator answering you only hear shhhhh on the other side. So I tried sending and email. Get a response that it is insurance and will be reversed. Didn't happen. Now they are telling me they will black list me. What do I need to do to close this account then? Horrible service