This plugin disables the visual editor for all users across the whole site. You could also remove the wp-includes/js/tinymce/ directory.

This plugin disables the visual editor for all users. You may allow the use of the Visual Editor for Admins if you would like. This plugin leaves the editor in place for posts and pages as well as custom posts types but only has the "Text" editor option for users to utilize. Very handy for developers who do not want other users changing HTML text when writing in the Visual elements. Plugin, on deactvation, will update usermeta and reset the database back to 'true' for meta_value 'rich_edting' and clears options so plugin will not linger after uninstall.