Unhappy with ADT Bloemfontein Branch

I have been a client at ADT BFN since 2014, I decided to stop my account with ADT on the 16\11\2019, I paid them the annual service fee including my monthly subscription fee upon my cancellation, to my surprise I have been harrassed and bul**** by ADT ever since, now ADT has listed me at Norman Basset attorneys saying I am owing them an amount of R400 and something, ADT had never sent me an invoice of that amount, no sms, no correspondence was made with me with regards to this amount, the next thig I receive a call from the attorneys, I called ADT BFN to understand why and for what is that amount, they couldn't answer me, I will not allow ADT to mess up with my credit status,for something I know nothing about, I have didputed this with Norman Besset attorneys and expect ADT BFN to remove this amount on my name.