Visualisation of the available hooks: https://wordpress.stackexchange.com/questions/39028/hook-filter-before-and-after-comments

comment_text hook

  Turning Off Commenting For New Posts Settings > Discussion > uncheck "Allow people to post comments on new articles" To turn off commenting for already publish posts Posts > select them all and "Edit" under bulk actions and hit apply > choose "do not allow" next to comments and hit update posts.