harassed by debt collectors for settled account

I am constantly harassed by capfin debt collectors to pay R450 which they claim is outstanding on the repayment of a loan I took out last year. I have settles the outstanding debt in December 2014 where I was given a settlement amount by the debt collector at the time, however 3 months back I started receiving calls from another capfin debt collector claiming that I still owe capfin R450 from the initial loan that I settled. What I do not understand is that I was given a settlement amount and I paid the amount stated in full, so I do not understand the constant harassment which is also affecting my credit record very badly as I have been listed on ITC by your debt collectors as a non payer. Please sort this out as it is really affecting my credit history.