単純にassignment/mod_form.phpを
if ($type != 'offline') {
$ynoptions = array( 0 => get_string('no'), 1 => get_string('yes'));
$mform->addElement('select', 'preventlate', get_string('preventlate', 'assignment'), $ynoptions);
$mform->setDefault('preventlate', 0);
}
すれば良いのですね。...しないけど