Dave Ferguson has created a plugin that secures pages or posts if you set a custom field while writing the post/page. In addition, it will only secure them if you are not logged in or you have a role that is not allowed reading the post/page. You can read more and download from Dave's blog.
Paul Klinkenberg has created a plugin that uses Project Honeypot to block possible spam. While Honeypot is also used by the bundled cfformprotect plugin, this plugin will completely block the page (not only the comment post) to a possible spammer. You can read more at his blog.