Last year, Google released a feature that allowed you to block sites from appearing in your search results. After clicking a result and going back to the search results page, Google displayed a special link next to the result for blocking the entire domain.
The feature no longer seems to work: the "block" link is no longer displayed, the preferences page doesn't mention the feature and the blocked domains still appear in Google's results. The page that allows you to manage blocked sites is still available.
"When you're signed in to Google, you can block a specific website from appearing in your future search results. This is a helpful option when you encounter a site that you don't like and whose pages you want to remove from your future results. If you change your mind, you can unblock the site at any point," explains a help center page.
Update: Google's Matt Cutts says that this could be a temporary issue. "The right people are looking at what needs to happen to re-enable this, but it might take some time."