Check out the official documentation for working with comments in WordPress here.
To turn off comments completely, you will need to disable commenting under→ by unchecking the “ ” option.
To turn off comments for single posts or pages, click on Screen Options in the upper-right and ensure the Discussion box is checked.
Scroll to the bottom of the page and uncheck Allow Comments and Allow Trackbacks under the Discussion box:
If you need to disable comments on an existing post or page, you may find it easier to hover over the post title in the index and choose, then uncheck Allow Comments.
You can do this for several posts/pages at once by checking them off (or check the box in the title bar to select all) and choose Edit from the Bulk Edit menu, then select Do Not Allow in the Comments option.
There are a few 3rd Party Plugins you may install to manage disabling of comments on all pages, all posts or specific post types. You can find them by going to→ and search for Disable Comments. Review the available plugins for an updated one that will do what you want.
Check out our WordPress video guides for a detailed walkthrough of working with comments: