You can block specific members from bidding on, or buying, your items by adding their ID to your blocked bidder list. The easiest way to access it is via the site map, or by bookmarking the link: http://pages.ebay.com.au/services/buyandsell/biddermanagement.html
If a member changes their user ID after you add it to your list, it is automatically updated in the list, so you don't need to worry about them being able to bid. It is also a breach of eBay policy to use other IDs to circumvent a seller's block, so if that ever happens, be sure to report them to eBay.
