Removing your entire website using a robots.txt file
You can use a robots.txt file to request that search engines remove your site and prevent robots from crawling it in the future. (It's important to note that if a robot discovers your site by other means - for example, by following a link to your URL from another site - your content may still appear in our index and our search results. To entirely prevent a page from being added to the Google index even if other sites link to it.
To prevent robots from crawling your site, place the following robots.txt file in your server root: User-agent: *
Disallow: /
To remove your site from Google only and prevent just Googlebot from crawling your site in the future, place the following robots.txt file in your server root: User-agent: Googlebot
Disallow: /
Source: Google Web Master Central