You can refund your client's payment or stop future payments by going to Billing-- payments-- and selecting the correct button!
Click the 3 circles next to the payment, and then click "Refund Payment".
A popup will appear. Type in the amount you want to refund, and click the refund button.
To stop automatic payments
Go to the first payment in the series, click the three circles, and select "Stop Upcoming Payments"