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<?php
//---------------------------------------------------
// # disable default dashboard widgets
//---------------------------------------------------

// disable default dashboard widgets
function disable_default_dashboard_widgets() {

  // disable default dashboard widgets
  //remove_meta_box('dashboard_right_now', 'dashboard', 'core');
  remove_meta_box('dashboard_activity', 'dashboard', 'core');
  remove_meta_box('dashboard_recent_comments', 'dashboard', 'core');
  remove_meta_box('dashboard_incoming_links', 'dashboard', 'core');
  remove_meta_box('dashboard_plugins', 'dashboard', 'core');

  remove_meta_box('dashboard_quick_press', 'dashboard', 'core');
  remove_meta_box('dashboard_recent_drafts', 'dashboard', 'core');
  remove_meta_box('dashboard_primary', 'dashboard', 'core');
  remove_meta_box('dashboard_secondary', 'dashboard', 'core');

  // disable iThemes Security dashboard widget
  remove_meta_box('itsec-dashboard-widget', 'dashboard', 'normal');
}
add_action('admin_menu', 'disable_default_dashboard_widgets');
?>


To disable plugin dashboard widgets get the id of the div that the widget is wrapped in and plug it in as in the example of the iThemes Security widget above