Can a bidder be blocked if the item is still active, their feedback is poor they are a non payer
Cancel their bids and block them.
Click on the Site Map text link at the bottom of the page, the links you need are both under Selling Activities.
Yes, block them here
And set your buyer blocks here to prevent non payers bidding
The reason that they are able to bid is that those sellers who left bad feedback are not wel-informed.
Sellers cannot leave negative comments for buyers "To Warn Otheres"
THey have done you no favours by letting the buyers off the hook.
If they had opened Non Paying Buyer disputes then the buyers would have strikes on their account which the second link above is for - it protects you from such timewasters