You block particular buyers by adding their ID to your Blocked Bidder/Buyer List.
You block (or limit) certain classes of buyer by setting your Buyer Requirements, especially those with 2 or more unpaid cancellations in the past 12 months.
Go to the Site Map (link at the bottom of this and most ebay.com pages) Sell Section (use the shortcut at the top--it's a long scroll down) and you'll find the link to that and several other useful seller tools. The Buyer Requirements are at the bottom of the page you are first taken to when you click the link to the BB/BL.
