Scott Nelle

WordPress lets you set a number of days after which comments will be unavailable for individual posts. This plugin will allow you to keep commenting open on specific posts.
Once the plugin is enabled:
<ul>
<li>Set a comment deadline in Settings > Discussion using the “Automatically Close Comments…” option. Closing comments on old posts is a great way to avoid spam comments.</li>
<li>Edit an individual post on which you’d like to keep discussion open indefinitely and check the Override Comment Deadline box.</li>
</ul>

Override Comment Deadline

Allows you to enable comments on posts which are older than the comment deadline set in discussion settings.

wpsimpletools

Completely disables comments functionality from backend and frontend: 
* Hides existing comments 
* Close comments on the front-end 
* Disable support for comments, trackbacks and ping 
* Redirects any user trying to access comments page 
* Removes comments metabox from dashboard 
* Removes comments links from admin bar 
* Removes comments page in menu 
* Completely disables comments API

Disable Comments

The type of plugin I just love... does exactly what it says, no configuration necessary, source readable and understandable in minutes.

Simple, effective, no config ✅

Completely disables comments functionality from backend and frontend. Just install it, nothing to configure!

Override Comment Deadline vs Disable Comments