How to give confirmation for deletion through link?
Don't bother testing the value of confirm, the user is only given a choice to submit or go back (with cancel link).

function products_delete_confirm_submit($form, &amp;$form_state) { 
  $param = $form_state['values']['_product'];
  drupal_set_message(t('Product ' .$param.' will get deleted.'));                             
  db_query('DELETE FROM {content_type_usergroup} WHERE vid = %d', $param);
  drupal_set_message(t('usergroup has been deleted.'));          
  $form_state['redirect'] = 'mailusers';
}


Also, your access callback is set to TRUE, which bypass all permission checking.

Final answer

It should be products_delete_confirm(&amp;$form_state, $product_code), see the example at https://www.drupal.org/node/751826#creating, you were messing with your form structure.