It comes as a feature after your first search query... Say you would want to block http://www.w3.org search results from the query "Owl", you will have to do a first search on OWL... Go to each of the results, on a click back to Google search home page from each result, a new feature "Block all w3.org results" appears next to cached and similar links.. On clicking "Block all w3.org results" link, the future results shall not be from "w3.org". The link appears even though the user has not signed in, but blocking requires the user to sign...