Here is how:

• To disallow comments in all posts and pages, do the following steps, this applies to new posts that you will publish from now on
  • Settings >> Discussion >> uncheck “Allow people to post comments on new articles”
• But what about all of existing posts and page, here is how to disable comments from already publish posts
  • Posts (or Pages) >> select them all and “Edit” under bulk actions and hit apply >> choose “do not allow” next to “Comments” and then hit update posts.