Google lets users block sites from search results
When Google hides a result you've blocked, you'll see a "blocked results" message at the top of the screen, with options to show the blocked results or manage your list of blocked sites. From the management screen, you can manually block sites without running searches for each one.
If these features look familiar, that's probably because Google has been experimenting with them using a Chrome extension called Personal Blocklist. All of Personal Blocklist's features have now been integrated into Google search, but you may still want to use it if you've already blocked a lot of sites and don't want to block them again, or if you want to block sites without being signed into Google.
Google has made no promises that blocked sites will be taken into account in future search algorithms, but they have said that they'll consider the option, and implement it if the blocked site data appears to be useful. Content farms should be quaking in their boots right about now.