how to disable all comment sections on all the pages?

To disable the comment section in all pages. You can log in to Dashboard > go to posts > post to be altered > quick edit.
You will see a check box for ‘allow comments’. Uncheck the box.
See the screenshot:
 
Also, you can use code to remove the comment section from all the pages:
– add the following to the “Header Script”

<style>
.single-post  #comments {
    display: none;
}
</style>

Hope this helps!