Here's an extract from Ebay website on blocking (i.e. barring) bidders. Hope it helps.
By adding buyer requirements to your listing, you can stop members from bidding on or buying your items based on certain criteria. You can block buyers who:
* Are in countries you don't want to send items to.
* Have a negative Feedback score.
* Have unpaid item cases recorded against them.
* May bid on several items and not pay for them.
* Don't have a PayPal account.
* Have breached eBay policies.
To add buyer requirements:
* Go to Site Preferences in My eBay.
* In the Buyer Requirements section, click Show, and then click Edit.
* Select your requirements, and the click Submit.
If you don't want to sell to certain bidders and buyers, you can put them on your blocked list. They won't be able to bid on or buy your items unless you remove them from the list. You can block up to 5,000 users.
To add people to your blocked list:
* Go to the Blocked bidder list page.
* Enter the username(s) you want to block, and then click Submit
.