Many times even after adding the meta noindex tag to the header of all pages on a site may not cause your website to be deindexed from Google. As it is up to meta robots to see the tag and decide whether or not to follow your instruction, it is important to know the other ways in which a site can deindexed.
1. Use Robots.txt
You can use the robots.txt
file for your website, this will block Google's bots from crawling your website. Add the following code to the site’s robots.txt for blocking Google's bots:User-agent: *