Close comments on all posts

For new posts: You can set comments closed as the default by going to Settings->Discussion and uncheck Allow people to post comments on new articles.

For existing posts: If you have access to your MySQL database, you can run this SQL code on the wp_posts table to change all posts to “closed” for new comments. (If you changed the prefix of the tables in your config file, look for the prefix_posts table.)

UPDATE wp_posts SET comment_status='closed'