First go to the main Ebay page 
(http://www.ebay.com). Look at the upper right 
hand side of the screen; you should see a link 
saying "my ebay" just above the "search" 
and "help" boxes. Click on this link, and if 
necessary, enter your log-in info when prompted. 
You should now be at your "my ebay" page. Click 
on the link that says "selling" - should be the 
second choice from the left side of the screen. 
On the next page, scroll down to the bottom and 
look under the heading "selling related links". 
In the far left column, under "managing your 
auctions" you should see a link for "block or pre-
approve certain bidders". Click on this link. On 
the next page, click on the "continue" button 
next to "add an ebay user to my blocked list". In 
the form box on the next page, enter the ebay 
user IDs or email addresses of the person(s) you 
want to block. Click "submit" and you're done.
Hope this helps you.