turn of auto payment settings here
ad delete card details so no payments can happen https://giffgaff.com/profile/payment-details
asak an agent for refund if not used it up for charges made this month by click here
tell an agent and they will deal with it and sort with in 24hours
explain that you dont use and it topped up itself and if they will refund as a goodwill gesture for the charge this month as you have not used. they can see on your account you havent used it and may refund.