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.

Hello All,, lm a bit of a comercial seller on feebay,,holden parts,  user name is FAIRLANELTD just incase you wanna have a gig,,been on ebay for a while,,and agree there has been a big influx of dickheads, l dont know where they are coming from..maybe from the boats,,he he he,,

Just a note,, you CAN block bidders,,with only under 3 feedbacks or less,, you just go to your account, then select selling preferences! will be there somewhere,,as l did that to stop the Zero bidders, as most are non payers,,if they are genuine,,they will email you.. l have lots of machines,,just cant bring mayself to sell them,,to hard to find.. good luck.