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. You can also set an option to block some messages (but not all, depends on the circumstances especially whether you have had a recent transaction or whether you sent them a message or replied to one of theirs) from blocked buyers there.
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.
