Worst service lack of intergrity

I requested for settlement letter on 05 March 2018 and I settled the account and requested for the account to be closed and Discovery told me that the account can't be closed because there's an amount owing in my account. My bank statement showed credit balance due to me I quote from their correspondence "On 19 March 2018 your statement reconciliation showed R98.94 due to you as per the transactions that had taken place until then. The R 81.05 that was still outstanding was the interest. You had R180 interest outstanding on 19 March 2018, thus R180 - R98.94 (credit balance) = R 81.06 - 1c(interest earned for the credit balance) = R81.05. " I enquired why was I charged interest on 19 March whereas I sent a close account on 08 March while the account was on credit. Even from their recon it's not clear. I have been asking them to reverse the interest billed on 19 March but they insisting on me paying. Worst service ever