To refund a membership, go to Statistics, then Transactions in the admin panel. You will find a list of all transactions for your venue. Find the membership purchase and select Refund. This refunds the amount to the original payment method.
Refunding a membership does not remove the membership from the customer. To remove the membership, go to the Customers page, search for the customer, click their name to open their customer card, and find the membership section. Here you can remove the membership from the customer.