Hi there and thank you for your question,
I will try to assist you with your legal question but please feel free to ask as many follow up questions as you like until you are 100% satisfied.
There is unfortuantely no legal obligation on FNB to take the interest off the amount. They were also entitled (if there was an outstanding amount) to list you as a bad payer. Unfortunately they can do that.
The only way that they will take the interest off is by agreement. You should write to them and request them to remove (or waive) the interest on the basis that you can't afford to pay it.
Hopefully they will come to the party and assist you!
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Good luck and best regards,