How do I add a buyer to my block list?

If you've had a bad experience with a buyer, you can block them from buying from you again using your Blocked Buyer/Bidder List

Where to find it - click the link above, or go to the eBay Site Map then under Selling Activities/Manage Bidders choose 'Add an eBay member to my Blocked Bidder/Buyer List', then simply type in the buyer's eBay ID, and click submit.