If you are fed up with this stupid message, then go to /drigg/drigg/helpers.inc and comment the line:
drupal_set_message(t('Following not exiting Uids mentioned in "submit as user" field has been removed: ').$invalid_uids);
like this:
#drupal_set_message(t('Following not exiting Uids mentioned in "submit as user" field has been removed: ').$invalid_uids);
And now you can live again!