Account paid yet I'm still being harassed

I was handed over by Fynnland Senior Primary to Credit worx for a amount of R3200 at the end of November 2013 I then made payment at the school for the full amount on 15 January 2014 and notified the company named. I was then contacted by Norman BISSET and associates regarding this account. I have repeatedly sent the proof of payment to them yet til date I am harassed by call centre agents for this account. When I inform them yet payment was made they tell me that they do see the proof of payment attached and will send an email to the administrator to close the account. It has been 2 years since I paid the full amount at the school and am still being hounded for this account. They have no proper explanation as to why this account hasn't been closed and as a result has cost me having to Phone in and try to get help. Even their manager whom named is Ingrid who I have spoken to countless amounts of times hasn't helped me. This is now affecting my credit rating and ITC listing