Revolving Credit- FNB refuses to reduce available amount

In 2018 upon logging into my internet banking I was offered a 190k revolving credit plan. I accepted. However I have been trying since March this year to have them drop the available amount to zero so they can recalculate the repayments based only on outstanding credit. But to no avail. I went into branch (Mall of Africa) and they said they could not assist, that I would need to call the call center. I have done this several times and spoken to several consultants. They told me they requested it but don’t have the authority to do it. That I would receive an email advising of the changes. Months have gone by and nothing done yet. I called again, same process. It seems they are trying to keep me paying them more interest- clever FNB. But I just need this sorted now! And quickly please