You are responsible for all charges that have already been incurred on your credit card, for current and previous months. To make sure everyone is treated equally, no exceptions will be made and no refunds for previous charges are available.
Your billing cycle begins the day you sign up for one of our premium plans, and cancellation of that subscription before next date of renewal will ensure you won't be charged further. Cancellation after incurring a charge will stop your future billing, but no refunds will be issued.
For instructions on cancelling a current subscription, check out this article.