How to remove a domain from google search results?

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]