There is no switch type control to turn on or turn off commenting in SharePoint blogs. But there are some tricks using which we can get same kind of control over comments.Once you create a blogging site in SharePoint it creates some lists by default . There is also one list for comments. Whenever user logs a comment for any post, it is stored in this list.
We can update settings for this list and control how commenting works for blogs.
Comments will not be displayed unless it is approved
Every comment should be approved before it is displayed for other users. It is an ideal scenario for most of the cases. You can achieve this by updating the versioning setting for Comments list.
After this, update Item-level permissions for comments list to not let users update comments from others.
For Read access : select ‘Read all items’ to let users read all the approved comments.
For ‘Create and Edit access’ : Select ‘Create items and edit items that were created by the user. By setting this, you are not allowing users to edit/update other users comments.
Make sure that users have Contribute access to Comments list.Also it is better to provide contribute but not delete access to end users to stop them from deleting other comments.
You can turn off commenting in SharePoint blogs by updating the permissions for ‘Comments’ list. Edit the permissions, stop inheriting and remove all permissions. This should prevent end users from commenting.