Is there a way to Disable the visual editor for all users without logging in and setting each user's setting in the admin panel, preferable some way to do it with code in the functions file?
The easiest way seems to be to use the Disable Visual Editor Plugin.
The plugin also recommends that you remove the
wp-includes/js/tinymce/ directory, but will require maintenance every upgrade.
Not sure if this still works. Preferably, drop this inside your theme's functions.php:
add_filter( 'user_can_richedit' , '__return_false', 50 );