There is currently no way to block zero feedback bidders entirely on ebay. You can block bidders with a negative feedback rating, which we do, but there is no way to block zero feedback bidders.
http://answercenter.ebay.com/question/Miscellaneous/Block-Zero-Feedback/900354696
The only thing we could do to guarantee no zero feedback bidders would be to make our auctions closed to all but "approved bidders". This would require ALL bidders to contact us and get put on a white list before bidding. We are not inclined to pursue this as it seems like it would be inconveniencing the established and legitimate bidders. It would also present a problem for last minute bidders or snipers.
Despite the stigma some attach to new ebayers, we have had many successful transactions with new ebayers (zero feedback bidders) so we are not inclined to put a blanket ban on them, even if that were possible.
As was said previously in this thread "We cannot simply block or retract bids simply because the bidder has zero feedback. One of our initiatives with EBAY is to bring new bidders to the site and this would be contradictory to the larger marketplace we are trying to create. However, we do actively and responsibly police our auctions to ensure the integrity of them."
In our experience Illegitimate bidders are very rare in our auctions. We are working closely with ebay to streamline verification for new bidders and not inconvenience established bidders. It may take some time to figure out the right process for these rare circumstances but we are confident it will be worked out. In the meantime, we hope that bidders are assured in the fact that we are adamant about running safe and honest auctions. We do police our auctions and will take action based on our findings.