How do I handle a credit card late fee that was not late?

Capital One charged a late fee last month which in reality was not late. I called them and they agreed and said the late fee would be taken off and we did not have to pay it. This month they charged us a late fee stating that the fee was because we did not pay the late fee last month; although on our bill it also credited for last month's late fee. In other words last month they charged a late fee, we call... we pay minimum amount minus late fee. Then this month the bill shows credit of the late fee, however, they again charged us another late fee, because we had not paid last month's late fee. We called them and they refuse to take the late fee off and that we HAVE TO PAY it regardless.
Is this at all legal? What can we do?
Please help,
Thank you
Lee