If you can use a userscript, I found this to be an excellent replacement for functionality that used to be provided by Personal Blocklist (now discontinued):
https://www.jeffersonscher.com/gm/google-hit-hider/
Answer from Keyslinger on Stack ExchangeIs there a way to remove sites permanently from (my personal) Google search results? - Web Applications Stack Exchange
How to remove a domain from google search results? - Webmasters Stack Exchange
Remove domain from Google Search.
Way to block a domain from Google search without blocking all images from Google search
Videos
If you can use a userscript, I found this to be an excellent replacement for functionality that used to be provided by Personal Blocklist (now discontinued):
https://www.jeffersonscher.com/gm/google-hit-hider/
For Chrome users, the official Personal Blocklist Chrome extension can remove sites from your results. Unlike the one referenced in Bernhard Hofmann's answer, this is an official extension developed by Google.
Google used to have built-in support for this, but it has removed this feature. If you used this feature, you can still download your block list as a text file.
You can verify that you are the owner of the site by using DNS verification: http://googlewebmastercentral.blogspot.com/2010/03/dns-verification-ftw.html
Once you've done that you can request removal of the whole site http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=164734 but they may ignore it because you haven't got a robots.txt entry.
If you do nothing eventually search engines will give up and drop your site from their indices as they always get errors when accessing this domain.
Google and other search engines look at the HTTP status code that gets returned with any request. The most commonly known ones are '404' for page not found, '403' for access denied, or '301' for content that has permanently moved.
There is a HTTP status for 'content gone', it's 410. This is often used if content has been removed permanently with no new address.
Your best bet is to set your domain's DNS to some server somewhere, and configure Apache to return '410' for every request. You could do that by enabling mod_rewrite and sticking this rule in your .htaccess file:
RewriteEngine on
RewriteRule (.*) - [G,R=410]
Hey folks.
We had some of our test environments being indexed by Google.
We added X-Robots-Tag: noindex, nofollow.
Requested via the Google Search Console a Cache clear, which went through successfully.
BUT, for some reason, those test environments are still visible in Google Search.
Inspecting the URL (https://sub.domain.foo) shows it being in Google. Requesting Indexing fails at the Live Test, because the website the noindex option from the X-Robots-Tag is detected...
There is also another subdomain, which clearly says "URL is not on Google"... yet it still appears in searches. Setting basic auth (thus giving 401 to the bots) also does not remove it from the searches after almost a week.