Spam fighting is certainly a necessity for the growth of successful online markets. Google is proposing a new approach to fighting spam, which gives control to users. Essentially this approach would allow users to remove or blacklist domains within the Google search results. Google has already tried to do something similar with SearchWiki, but the approach was limited to the page level. This supposed new approach would be domain-wide.
One of our concerns is that this DIY blacklisting approach would be abused by competitors or even spammers themselves to squash strong competitors in search rankings. Besides automated script/robot abuse of this new function, we see a potential problem with factory-style "SEO-spam" that try to downgrade competitor websites using multiple accounts, users and IP addresses but with a single protocol. We are not so much concerned with the idea of a de-centralized authority for spam defense, but we are concerned that spam defense itself may become the object of abuse by those who the approach was intended to fight against.
Source: Google Blog, 1/21/2011, Google search and search engine spam