Being black listed wrongfully

I had a loan with SA multi loan who after paying it off, deysaid its not finished, I was garnished and paid it for two years. I then started to be harrased by shapiro who said I still owe sa multiloan, I am consta ntly sent sms's when I phone back I was then insulted by the call centre consultant,who told me I must get the paid up letter from sa multiloan and give it to them and stop phoning them. They told me they will go ahead and blacklist me. I am constantly being harrased by them.