Try putting him/her on your Blocked Bidder/Buyer List and in your Buyer Requirements click the box to block messages from blocked buyers. This is not 100% effective as there are situations where the block does not apply (in a recent transaction, if you have messaged or replied to them recently) and some question whether it applies to those on the BB/BL as opposed to those who are blocked by a Buyer Requirement.
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 those (Buyer Requirements are at the bottom of the instruction page the link to the BB/BL first takes you to) and several other useful seller tools.
