/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_page_node_form_alter(&$form) {
  mymodule_disable_wysiwyg($form);
}
 
/**
 * Disable wysiwyg in all fields
 */
function mymodule_disable_wysiwyg(&$form) {
  foreach (element_children($form) as $element) {
    if (isset($form[$element]['#type']) && $form[$element]['#type'] == 'text_format') {
      $form[$element]['#wysiwyg'] = FALSE;
    }
    else {
      mymodule_disable_wysiwyg($form[$element]);
    }
  }
}

Drupal 7