Stop Google from indexing

robots.txt

User-agent: *
Disallow: /

this will block all search bots from indexing.

for more info see: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=40360

There are several way to stop crawlers including Google to stop crawling and indexing your website.

At server level through header

Header set X-Robots-Tag "noindex, nofollow"

At root domain level through robots.txt file

User-agent: *
Disallow: /

At page level through robots meta tag

<meta name="robots" content="nofollow" />

However, I must say if your website has outdated and not existing pages/urls then you should wait for sometime Google will automatically deindex those urls in next crawl - read https://support.google.com/webmasters/answer/1663419?hl=en

Is there a way to stop Google from indexing a site?

To stop Google from crawling simply add the following meta tag to the head of every page:

<meta name="googlebot" content="noindex, nofollow">