MySQL query to disable comments on WordPress posts/pages

[admin]

The settings in WordPress to disable comments on posts (and pages) are not retroactive.
Only new posts/pages will have commenting disabled.

To disable comments on all existing pages and posts, run this MySQL query from your database manager.

UPDATE wp_posts SET comment_status="closed";

Many people use teh free (but lousy) phpMyAdmin, but we suggest the $99 SQLyog Enterprise Manager.
It works better, and it makes like easier.

Hope this helps.