Cell C account fully paid but Nimble fails to clear my name

I have been sending them emails and proof of payment in an attempt of solving the issue of Cell C account that I have fully paid. every time I send the proof they disappear and they will only re-appear after few months again with the different consultant. They are now not responding to any correspondence that I'm sending, as a result my credit score is being compromised for the account that i paid few years ago.