Comments are enabled on new posts and pages by default. You can change the default for new posts or pages, as well as enable/disable comments on posts or pages you’ve already published.
Start by changing the defaults for new posts or pages in the Settings → Discussion menu in your blog’s dashboard.
In the first section − Default article settings − disable the option Allow people to post comments on new articles.
This setting may be overridden for individual articles. To do it, check the option Allow Comments when editing your post or adding a new one.
To disable comments for all past entries / posts, you can bulk edit them, using the Quick Edit option in the Posts section of your blog’s dashboard.
You can also en to turn comments off automatically after a certain number of days. For more information about this, read this article