and you cant block their bids (bidders with zero or better are permitted to bid on anyones auction), just because you say so in the listing doesnt stop them for bidding, and actually stating so is against ebay rules, you can have your listing pulled for unfair/unreasonable seller demands

You can cancel anyone's bids, but at the end of auction you cant do it quick enough.

At best you could block usernames with low/zero feedback using the block bidder feature in your account settings. But they will just create new usernames.