This plugin allows administrators to globally disable comments on any post type (posts, pages, attachments, etc.) so that these settings cannot be overridden for individual posts. It also removes all comment-related fields from edit and quick-edit screens. On multisite installations, it can be used to disable comments on the entire network.
Additionally, comment-related items can be removed from the Dashboard, Widgets, the Admin Menu and the Admin Bar.
Important note: Use this plugin if you don’t want comments at all on your site (or on certain post types). Don’t use it if you want to selectively disable comments on individual posts – WordPress lets you do that anyway. If you don’t know how to disable comments on individual posts, there are instructions in the FAQ.
Get this plugin here.