function MYMODULE_form_alter (&$form, &$form_state, $form_id)
{
  // comments
  // http://api.drupal.org/api/drupal/modules--comment--comment.module/function/comment_form/6

  if ($form['#id'] == 'comment-form') {
    $form['comment_filter']['format'] = array(); // nuke wysiwyg from comments
  }
}

/**
 * Implements hook_comment_form_alter().
 */
function comment_form_alter(&$form, &$form_state, &$form_id) {
  $form['comment_body']['#after_build'][] = 'MYMODULE_customize_comment_form';
}

/**
 * Comment body after-build function.
 */
function MYMODULE_customize_comment_form(&$form) {
  // Hide text-format selector in comment form and fix format to filtered_html
  $form[LANGUAGE_NONE][0]['format']['format']['#value'] = 'filtered_html';
  $form[LANGUAGE_NONE][0]['format']['format']['#access'] = FALSE;
}