To disable comment on all new post do the following.
Settings -> Discussion -> uncheck “Allow people to post comments on new articles” -> Hit “Save Changes” in the bottom of the page.
Note that these settings may be overridden for individual articles so it’s flexible.
To disable comments from already published posts do the following:
Posts -> select all and “Edit” under bulk actions and hit apply -> choose “do not allow” next to comments and hit update posts.