If you want to make sure it does not get indexed and have access to mod_rewrite, try:
RewriteRule !^no-index\.html http://example.com/no-index.html [R=301,L]
Then make no-index.html the following:
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<meta name="robots" content="noindex,nofollow,noarchive" />
The RewriteRule will redirect any request that is not for example.com/no-index.html to example.com/no-index.html. The meta tag will prevent no-index.html from being indexed,followed,archived, and the site will disappear from the SERPs.
You should be able to safely remove the robots.txt.
Another mod_rewrite alternative is:
RewriteRule .? - [F]
The preceding will serve a 'Forbidden' error any time any page is accessed. Basically, it says to *everyone* 'You do not have permission to access the site', and will cause it to be dropped from the indexes. (This one might be the easiest / most effective.)
DO NOT use either of these suggestions if you need to allow access to the site, because they will not allow any visitor (SE or human) to see it.