Use the drop-down menu to the left of the labels (Seller Hub/Orders/Shipping labels) to 'Void' the unused labels. Note that if they're USPS labels, it takes about 3 weeks to get a refund. Good luck.
And if they're not USPS postage labels then there's no need (or even any option) to void them because UPS will automatically cancel a label that's not used (iirc within 5 days) and FedEx doesn't bill until the piece is scanned into their system.
