function hide_user_columns($columns) {
    unset($columns['username']);
    unset($columns['role']);
    return $columns;
}
add_filter('manage_users_columns', 'hide_user_columns');
function disable_theme_functionality() {
    // 
    global $submenu;
    unset($submenu['themes.php'][5]);

    // 
    define('DISALLOW_FILE_EDIT', true);
}
add_action('admin_menu', 'disable_theme_functionality');
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