Exceptionally poor service trying to closer a credit card

On Tuesday the 18 January 2018 I requested a settlement letter from FNB for my gold credit card. After not receiving the letter i further requested a second settlement letter. I then received the settlement letter on the 25th January 2018 which stated that i owe R 11,666.21 which is valid until the 31 January 2018. A payment of R12,300.00 was made into the account in question. I then went into the FNB branch at Eastgate on Friday the 26 January 2018 where I was told to phone in and cancel my credit card/account over the phone (as you cant do that in branch). On the 26 January i made a purchase of R 300.00. Now keep in mind that i over paid my account by roughly R 600.00 and this transaction would be completely covered. I phoned on Monday the 29 January 2018 to close the account but the transaction had not cleared by then and was told to phone back today (31 January 2018). I have phone FNB three times today and have received zero help from their employees. The employees keep telling me i have an outstanding amount of just under R 341.00 (and a few cents) and will not assist me in closing my account until this amount has been paid. Now this is basic math for anyone to do so why am I expected to pay a further amount to clear my account when in actual fact my account has been overpaid in the first place. My available balance is current at R 11,768.00 - so how do i owe the bank money if my available balance is more than the settlement letter? Not to mention the amount of airtime i have had to purchase to resolve this unresolved issue. This is such poor service and i have never been so disappointed in FNB as i am today