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. Note that you cannot totally block low feedback buyers this way, but you can limit the number of items (x) anyone can buy or commit to buy from you in a 10 day period and then exempt those with y or more feedback score (eBay phrases this as only applying to the block to them but that confuses too many sellers who think it is a total block rather than a limit that doesn't apply until after they have gotten 1 item).
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.
