This extension is made to work on Ricardo.ch auction site. You can make a list of the nicknames you don't want to buy from.You can blacklist a seller on Ricardo so that you don't buy from them later by mistake.
Go to Options page and add the nickname. That simple.
Then on the product page, "Buy Now" and "Bid" buttons shall be disabled when the seller is in your blacklist.
It will also add extra "[BLOCKED]" text next to the blacklisted nickname.