To completely turn off commenting on an existing site, do the following:
STEP ONE: Macro Level
Go to the Dashboard, then Settings, then Discussion.
Turn off both check marks in “Email me whenever”.
In the same section, turn off all three boxes in “Default Article Settings.”
You could even turn on “Users must be registered and logged in to comment.”
STEP TWO: Posts Level
Then go Posts < All Posts.
Click on the button beside the Title in the listing of Posts. That will select all of them.
Then click on the drop down menu whose default option is “Bulk Actions.” From that menu, choose EDIT.
Then mouse over the first post. A little menu will appear. Choose “Quick Edit.”
This Quick Edit feature allows us to edit a post or page without actually opening it. It’s useful for changing things about the post or page, as opposed to changing content inside it.
In this Quick Edit area, make sure that Allow Coments and Allow Pings are both turned off.
STEP THREE: Pages Level
Finally, if you have commenting on any existing Pages, repeat the “posts” procedure above, but applied in your Pages section.