n3k4

To help your WordPress installation be more optimized for search engines, this plugin will include a ‘noindex’ meta tag in the header of any duplicate posts. 
Robots will not index any posts with matching title tags, which is a known indicator of negative SEO. This is very useful for automated posting and working with a large amount of posts.

Noindex Duplicate Posts

This plugin will automatically add a 'Noindex' meta tag to posts that are a duplicate determined by the slug and preexisting post names.

room34

This plugin is an add-on for Modern Tribe’s The Events Calendar plugin. It adds a “noindex” meta tag to the individual detail pages for events after their end date has passed, to prevent them from appearing in search engines like Google or Bing.
Note that this only takes effect the next time a search engine indexes your site, so past events may still appear in search results for a few days.

noindex Past Events for The Events Calendar

Automatically add a "noindex" meta tag to The Events Calendar's detail pages for past events, to prevent them from appearing in search engines.

Noindex Duplicate Posts vs noindex Past Events for The Events Calendar