NU DEBT BLACK LIST ME AFTER I MADE THE OUTSTANDING AMOUNT

Good day

Nu debt blacklisted me for amount of R996 for Optimi (Impac) homeschooling. I paid in December the outstanding amount of R800 now i get calls, emails, sms from Nu debt that i owe R996. I send all the proof of payments and emails to Optimi and Nu debt to state that i did pay the outstanding and still they send me emails, SMS. and when i reply they don't reply back to me. I am so fed up with them because they are ***********'s people working at that company and don't know what their doing.What can i do to let them stop. And get my name cleared? Their the wors company ever.