Disable all future comments
If you want to disable all WordPress comments on all future posts and pages by default, you can go to Settings menu in the WordPress dashboard then click Discussion and deselect "Allow people to post comments on new articles". You can enable comments on future articles. This just disables them by default so you don’t have to always perform that extra step.
Disable old comments
To turn off comments on previously published content you will need to click on Posts > All Posts in your dashboard (or Pages > All Pages). From there you can either select all the content by checking the box at the top of the column beside the word ’Title’ then selecting Edit and then hit Apply from the Bulk Action menu. What happens next is the Bulk Edit section expands and on the right hand side you will see a dropdown box labeled ‘Comments.’ From there all you will need to select ‘Do Not Allow’ and click update, and you have disabled comments on your previously published material.