Cell C a bad SP

I did an early settlement on my Cell C contract but later the Service provider claims that i'm owing them. The question is how can you close an account while the customer is still owing you. After a lot of up and downs i decided to pay them because at the end of a day my name would be ruined. i''m asking them a simple thing which is to update my ITC (Credit Vetting) but they cant do it. Most of the people working for Cell C don''t know what they are doing especially Legal department. I mean you don't have to think to do this, is just a sequence of steps and that's it. Cell C is no go area