Nu Debt will ruin your credit profile without cause

I have a contract with Telkom. My account was then in arrears in January. As soon as I was aware of this, I made payment to settle my account. Despite this I have been receiving one e-mail after the other from Nu Debt telling me that I owe money, that they will initiate legal proceedings for default judgment. It has been almost a month of such communication. Its been a month of having settled my account but just yesterday, I was informed that there has been a tracking alert that has been placed by Nu Debt with the credit bureaus which has an impact on my profile, whether big or small. Worst part? Nu Debt does not reply to e-mail or pick-up phone calls.