Generating interest on zero balance loan,

I took a loan with FNB in July and I settled it in September, then I called the loan department to close the account to avoid generating interest, I was told the account will automatically close after 30 days.

To date the loan account has not been closed and its generating interest, and I am told I must pay the interest generated on a zero balance or else they will black list my account.

Note I am not willing to pay the interest amount and if I am blacklisted as your consultant has claimed, I will gladly move my accounts to other banks