No, you do not have to accept a cancellation request.
In my personal opinion it's best to cancel no matter how annoying. If they've paid then you'll need to send the item, and may well receive an item not as described case when received, and receive back a damaged item.
If they haven't paid you need to ait 4 days and cancel order citing buyer has not paid, this gives them a strike.
Cancelling with Buyer Requests to cancel, refunds the buyer if they've paid and does not affect your seller performance, the quickest way to then relist your item.
Block them from returning add their ID here:
https://www.ebay.co.uk/bmgt/BuyerBlock?
@garden_happy
