They don't think twice to blacklist you even if you made arrangements with them

They have blacklisted me. I couldn't pay my last installment (this happened in 2017) & made arrangements to pay it in full with my tax money. As soon as I received my tax money a week later, I paid them in full. I even received a credit from them after their financial year end which they paid into my account. Now, in 2021, I realised that they have blacklisted me for the full outstanding amount (incl. the credit). When I phoned the offices in Pretoria, the lady that was supposed to asssist me, was so rude & suggested the whole time that I didn't pay on time, so this is my mistake. The financial manager which I made arrangements with in 2017, still promised me that this will not influence my credit score if I keep to my arrangements with them. I even have this in writing. This is the 2nd time that they've come up on my credit score; not even showing that I've paid them in full, but that I owe them money!!! It has also happened to a collegue of mine which I introduced to Pretorium Trust.