You can cancel a prepaid subscription order that has not yet been fulfilled.
If any scheduled items have been fulfilled, then you can't cancel the order, but you can issue full and partial refunds for fulfilled and scheduled items.
If your customer cancels an order, then you might still need to refund the order manually. A message appears on an order when this happens.
Note: When you refund an order from the Orders page, you might need to cancel the order in Ongoing as well.