You can issue a cancellation via the dropdown menu for the item on your orders page. Use the reason "Something wrong with buyer's address". The refund will be issued in the process.
And you can cancel the label to get a refund. Go to orders, select Postage Labels on the left-hand side, then go to the actions dropdown menu next to the label and select Void Label.
Send message to buyer to explain. Either invite them to correct their address and buy again. Or just put their ID on your blocked bidder list:
https://www.ebay.co.uk/bmgt/BuyerBlock