1. login to the WordPress dashboard
2. Form the left hand-sidebar choose posts or pages.
3. Open the page or the post which you want to disable comments from it, in edit mode
4. From the screen, option activate the "Discussion" option
5.scroll to the bottom of the page and find the discussion section
6. Disable "Allow comments" and "Allow trackbacks and pingbacks on this page"
If the "comments are closed" section appears on the page or the post, use following CSS into the site settings >> general >> custom CSS