They're not, at least not by eBay. You can manually cancel their bids as they come in, but should one sneak through at the last moment, eBay will issue you a defect if you cancel the transaction after the auction ends. Too many defects, no more eBay selling account.
And, for those who are using the far more lucrative method of fixed price instead of auction, there is no bid cancellation or accepted cancellation for the non-supported requirement of 0-4 feedback.
In being penny "wise" in protecting yourself from the supposed increase in risk of zero feedback buyers (a risk that I have not personally seen in 17 years of selling on eBay), you are perhaps being pound foolish by leaving money on the table by running auctions versus fixed price listings.
I would not recommend fooling around in an eBay gray area that's more likely to lead to trouble than avoid it. I also wouldn't recommend auctions over fixed price listings except in very specific cases (a very rare and very desirable item that will surely attract a lot of buyers, or a very flavor-of-the-month very rare item -- new style iPad, hottest toy this Christmas that's out of stock everywhere, etc.)