you really have only 3 choices:
1.
<meta name="googlebot" content="noindex,nofollow">
"don't look, google"
(links wouldn't flow to the rest of the site; url/content won't appear in the index)
2.
User-Agent: googlebot
Disallow: /a.html
"you're not allowed to look, google"
(links wouldn't flow to the rest of the site; url might appear in the index, but not content)
3.
RewriteCond %{HTTP_USER_AGENT} googlebot [NC]
RewriteRule ^a\.html$ - [G]
"there's nothing to see, google"
(links wouldn't flow to the rest of the site; url/content won't appear in the index; user agent cloaking is risky)
it's not like there's anything you can do about google discovering the links to your domain, unless your link building effort includes preventing googlebot from discovering or following links to your site.