With Personal Blocklist installed, you can prevent particular web domains or hosts from appearing in your Google Search results.
While you are on the results page of Google, just click the “Block example.com” link next to a search result and all pages hosted on that domain will no longer appear in your future search results.
Internally, the add-on uses CSS to hide those domains in search results and your action is less likely affect the search rankings of that domain though the blocked URLs list is transmitted to Google.
If you wish to unblock a website, search for that domain in Google and toggle the “Show” link against the text “Some results were removed by the Personal Blocklist Chrome extension”. You now have an option to unblock that domain.