Interest charged but paid in FULL long before due date

4th call just made to the bank, to tell me that managers are in a meeting after being promised a call by end of day yesterday. interest is charged even though i pay the FULL amount. i have raved over discovery for so long and feel like a fool being part of this bank. the worst part is we wont be part of rewards next year if i close the account but we might just need to cut our losses and walk away before the interest charged gets more each month as i am already on R800 in interest owing to me and doubt i will see it