Cancelled services and Unauthorised debit order

These people gave me the absolute worst service for the past 12 months until they decided to cancel my service. I decided not to query why they cancelled because I was glad that relationship was over. They sent me an email to send their router back - which I did. A few days later they tried to debit my account for an amount of R2547!!!!!! For no good reason. I queried this with them and the response was "we haven't received the router; we will credit your account with R999 once the router is received and you are liable for other charges". This company should not be allowed to operate the country because they are fvcken ****** and rob people out of their hard-earned money. Now not only do I now have to carry the bank charges costs, but I paid for the courier myself. I am so tired of having to deal with their incompetence even when I am no longer their customer.