Not responding to e mail correspondence

Realised I was put on ITC when app**** for credit - R650 I have correspondence to the effect that the contrct was converted to pay as you go during 2013 Various e mails to faceless society of [Email Removed] not responded to They have all my contact details and still choose to ignore requests for proof thatI owe the money .To worsen the situation another faceless company DMC Debt Mangement (pty)Ltd choose to follow the same startegy as Virgin Mobile who gave them the instruction to collect the outstanding amount. How can companies like this do what tey want and choose to side step the issue .I had to pay R650 to get thecredit that I apl**** for , even When I disagreed that I owe the money .I need to het this cleared from my name ????