You can disable all the WordPress comments at a time using the wordpress discussion settings. Here is how to disable comments in WordPress Posts.
Go to Settings >> Discussion
Check – Automatically close comments on articles older than [X] days – Check the box and enter the number of days (e.g. 14 days) after which WordPress will automatically flag eligible posts so that no more comments are accepted. Note: If you want to completely disable comments make the number of days as zero(0).
Uncheck – Allow people to post comments on new articles.