Seth Alling

Up until recently, WordPress gave two options: You could either disable comments and trackbacks by default for all pages and posts, or you could have them active by default. In WordPress version 4.3, this finally changed so comments are always disabled on new pages.
While the new change makes it easier for many of the sites, it make it harder for people who need to get the reverse and enable comments on all pages, or if they need to change the default for a custom post type. This plugin allows you to choose whether comments are enabled or disabled by default on all new posts, pages and custom post types, while still giving the ability to individually enable comments on posts or pages.
Also, this plugin provides a way to quickly disable all comments or pingbacks for a specific custom post type. It directly interacts with your database to modify the status, so it is highly recommended that you backup your database first. There shouldn’t be any issues using this feature, but it’s always good to play it safe.
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No Page Comment

An admin interface to control the default comment and trackback settings on new posts, pages and custom post types.

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!

No Page Comment vs Disable Comments