Sick and tired of CellC credit department!!!!

After months of fighting, CellC finally sorted out my account and everything is up to date.....or so I thought.. Now I am receiving sms saying that the account has been handed over to the credit bureau for slow/non payment on the reff number of old contract... I'm not sure where the people in their credit department got their training, but seriously, they don't know what they are doing!!!! I seriously want fo cancel my contract with CellC as I do not have the time or energy to do my job as well as theirs!!!! How can I cancel my account with them please!!!!!!