Sellers can block certain bidders by adding the member I.D. to their blocked bidder list. Sellers can also set buyer requirements to block bidders who don't pay or have other policy violations on their account. Sellers can block bidders in countries to which they don't ship too. All of these blocks are automatic so a buyer can't bid/buy if they are blocked.
Last, sellers can cancel bids and block bidders while the listing is active if they don't feel comfortable with that bidder.
Sunny
"There cannot be a crisis next week. My schedule is already full."
Henry A. Kissinger