Irregular Finance Charge

I had paid up my credit card to a zero balance and month end a deduction was taken, when I enquired they said I requested it. The lady that assisted me didn't tell me I needed to cancel it once I paid up the credit card. Further in the month I see a finance charge on my credit card, when I enquired about it I was told it's a error on their part yet if I didn't call they would have left it!!! I was then told that I need to call back on Monday and tell them to reverse it! They took money that was not due and now it's my problem. Their service and staff is pathetic. If I had to owe them they would not wait.