From the eBay Home Page select Services/Buying & Selling. Halfway down the left column of the resulting page you'll come to "Bidder Management." Clicking on that gives you the following information intended for sellers: "Blocked Bidder/Buyer List — Set up a list of bidders/buyers who will no longer be able to bid on, or purchase, your listings. The list will apply to all your items. You can add or delete bidders/buyers at any time."
Below that little tidbit is an icon you can click on to add a buyer to your blocked bidder list. Maybe this is the "Banned Bidder" you're encountering. (Too bad they don't have a similar feature that would allow bidders/buyers to ban sellers from eBay.)
