Please post a link to the policy that states that requirement.A buyer can ask to cancel within the first hour after an auction ends. You are required to cancel. After an hour, it's at your discretion to cancel. However, if you've taken an unpaid item strike after the buyer has asked to cancel, eBay in all likelihood will remove it at the buyer's request.
AFAIK, once a seller has completed a contractual sale, the seller isn't obligated to agree to a cancellation.