...currently showing in the SEO results?
The other answers are more about proactively preventing the indexing of a (sub)domain (which is primarily what you are asking in the question) rather than actively removing your pages from the search results, which might be more what you are after, judging by your other question.
You still need to block your site in robots.txt and serve a noindex
meta tag (or
X-Robots-Tag HTTP response header), as stated in the other answers, but you also need to block access to your pages, returning a 404 or 410 status code.
You can read more about this on the Google Webmaster Tools help page:
Requirements for removing content
Once you have all these in place then you can use the Remove URLs tool in Google Webmaster Tools. However, this only applies to individual URLs, not an entire site, but it's a start. (Google states that using robots.txt, noindex and serving a 404 are the requirements in order to use the GWT removal tool.)
However, if you still want regular users to be able to access the site by typing the URL, then it's a problem - as your content is no longer available. You could password it, as Zistoloen suggests, however this will return a 403 (Forbidden) by default which you would need to override to return a 404/410. You could cloak your content, returning a 410 to the Googlebot and allowing everyone else to enter - but what about people clicking on results in the SERPs?
But if you want Google to remove your content in the fastest possible time then you need to remove it from the "Google" internet.