Actually it’s there within WordPress by Default.
If your WordPress editor is not showing it, then from the top left corner, click the Screen Options drop down menu. It’ll show some check boxes. Among those, there is one called Discussion. If you check that, then WordPress will show the option Allow Comments after the editor within Discussion Meta Box.
This is an author level option, so authors should be able to access this. If an author unchecks Allow Comments from the beginning, then no comment will be shown for that post. On the other hand, if an author unchecks this after some comments are received, then the older comments will be shown, but there will be no option to post new comments.