Kyle Gilman

This plugin does one thing and one thing only. It adds the text <code><meta name="robots" content="noindex"></code> to the header of any password-protected post. Google and all other respectable search engines will leave your password-protected pages out of their search results.

Noindex Password-Protected Posts

Disables search-engine indexing for all password-protected posts.

radgh

This simple and lightweight plugin adds a “Hide from search engines” checkbox above the publish box to pages. By 
ticking this box, a meta tag will be placed into the <code><head></code> section of your page specifying that robots should 
not index the page.
By default this only applies to pages. You may extend this functionality to posts or other custom post types with a small bit of code. See the FAQ’s tab for further instructions.

Noindex Pages

Ask search engines not to index individual pages by checking an option in the publish post box.

Noindex Password-Protected Posts vs Noindex Pages