Poor service for not confirming settlement and closing my account

I've settled my credit card account on the 29th March and have sent a request through to First National Bank Debt Review department to close my account and to send me a paid up letter confirming that the account is settled and close. Up to now my account is still active and I haven't received any letters from them and now interest was charged on my credit card account.

I feel this is unacceptable, as I've settled my account already on the 29th March and requested that the account should be closed and now due to their poor service, the account is now having a negative balance of interest that was charged on the account.

Can this please be finalised and be send through to me? I want to clear my name and can't continue with an account still open and having now a negative balance.