Go to the Site Map (link at the bottom of this and most ebay.com pages) Sell Section (use the shortcut at the top--it's a long scroll down) and you'll find the link to that (Blocked Bidders/Buyers List--note that it blocks buying fixed price listings also) and several other useful seller tools.
Also be aware that blocking a user is prospective only. It does not cancel any existing bids. Blocking and cancelling are two entirely separate actions.
