Google Allows Users To Block Your Site
For now its just an experiment and the results are restricted to the users personal search results - for which you need to register with Google and let them monitor your cache. Removing a site from your personal search results does NOT remove it from the index (not yet, anyway).
But it does grant searchers wth new-found powers to improve their own search experience, and gives Google the power (since they monitor personal search results) to manually check and remove spammy sites with even more efficiency.
Trust Google to improve the users experience by granting more power to them! 
Its no wonder they’re enthused by the possibility.
its use goes beyond removing spammy sites. You can remove links that may not be spammy but are just not relevant to the context in which you are doing the search… doing this can surface useful stuff that you may not have noticed because it was buried further down the serps.
Of course, the experiment has plenty of SEOs worried about competitors using this feature to get them banned from the general search index. Dan Thies warns Google:
Just don’t let anyone talk you into baking this into the regular search results without some serious thought about the concept of a “trusted user.” Community spam filtering works marginally well for email spam. Community filtering would work fine for search, for about 2 weeks, until someone starts a sweatshop in Jakarta to filter out their competitors… all of them.
But its seems unlikely for now, as Matt responds, because
you would definitely need to have a lot of trust in that data to use it for general search quality, and it’s early to even be thinking about that. For now, it’s an experiment to see how people like it.
Hopefully all it will do is just help Google filter out the spam better so the quality sites can thrive.
