I don't know of a way to specify that a specific buyer can be automatically barred from bidding on your items but there are several check boxes in the Additional Information selection on the Seller Form. Have a look and see if that helps.

I do know there's a manual way during the time the auction is going on, click on the # of bids and you can review who has placed a bid. In that screen, there is an option to "cancel bids" whereby you can select which bidder you want to cancel and that bid will be removed.

There's also the Unpaid Items drop down selection in your Items I have Sold section of your account.

Best of luck.

Steve