Aaron D. Campbell

Expire Comment Links was inspired by <a href="http://yoast.com" rel="nofollow ugc">Joost de Valk</a> 
in an article he wrote called 
<a href="http://www.searchcowboys.com/columns/764" rel="nofollow ugc">Comment links: an experiment</a>. 
It all started when Matt Cutts talked about how Google changed how it handles no 
follow links in his article on 
<a href="http://www.mattcutts.com/blog/pagerank-sculpting/" rel="nofollow ugc">PageRank Sculpting</a>.
No one knows for sure yet if this will be beneficial or not, but for those that 
want to be on the cutting edge and try this experiment for themselves, this 
plugin automates everything.
Requires PHP5.
You may also be interested in <a href="http://wpinformer.com" rel="nofollow ugc">WordPress tips and tricks at WordPress Informer</a> or general <a href="http://webdevnews.net" rel="nofollow ugc">Web Developer News</a>

Expire Comment Links

Allows you to stop displaying links for old comments. PHP 5+ required.

Marat Nepomnyashy

Overview:
At the time of this writing, the latest version of WordPress, version 4.7, 
 has functionality to discourage search engines from indexing the whole site, 
 but not functionality to discourage search engines from indexing only certain 
 specific pages and posts of the site.
Robots Meta Whiz is a WordPress plugin that allows site administrators to 
 deploy custom meta robots tags to discourage search engines from indexing, 
 following links from, caching, indexing images from, and / or have 
 additional custom restrictions on only for certain specifically-designated 
 pages and posts.
It is an easy way to discourage search engines from indexing only specific 
 pages / posts.
This only applies to search engine robots, and will not effect the site’s 
 local search functionality.
Technical summary:
Plugin works by hooking-in special logic into the action ‘wp_head’ to inject 
 the tag <code><meta name='robots' content='...'></code> with restriction directives for 
 the specified pages and posts.
Official project URLs:
https://wordpress.org/plugins/robotswhiz 
 https://github.com/maratbn/RobotsWhiz

Robots Meta Whiz

Easy way to discourage search engines from indexing only specific pages / posts with custom meta tags.

Expire Comment Links vs Robots Meta Whiz