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. Also there you can select to block messages from blocked buyers (not 100% effective as there are situations where eBay will not do so: if you were in a recent transaction or if you had messaged the other user possibly including replying to his/her message).
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.
If you mean something else please be specific.
