Here is a link to a page with a link that will take you to your ebay blocked buyer list. There, you can add or remove buyers from your blocked buyer list. Each member id must be seperated by a comma and a space.
http://pages.ebay.com/help/sell/manage_bidders_ov.html#blockYou can block up to 5000 user ids
Here is the list of bidders that I have blocked for whatever reason. Most are wannabe scammers.
0855jill, 123saveski, 2009-c-clement, amphiboy, armyofhades, beer_escamoso, blondiern1, ciabattoni2006, ebuyer2007es, gotsw, hummingjeep, jhtp01, jokabofe, keysdisease23, mooman_12, omedome, reminton.steele, shorbfamily, street-dragon, nadmunsos