Being charged interest on an account for no reason

I am being charged interest yet I settle my account every month before the payment due date. I queried it several times and no one can explain why I am being charged interest. This is day light ******* by this company. No one employed by this company is competent enough to understand my query and explain why I am bring charged interest. I asked to have the account closed and I was sent a settled figure for the interest. EDGARS ARE *******