Hi,
unfortunately this is not possible without a hard code change.
In file at /administrator/components/com_jsn/helpers/fields/selectlist.php at line 254 replace this code:
$where.='b.'.$db->quoteName($field->alias).' LIKE '.$db->quote('%"'.$opt.'"%').' OR '.'b.'.$db->quoteName($field->alias).' = '.$db->quote($opt).' OR ';
with this
$where.='(b.'.$db->quoteName($field->alias).' LIKE '.$db->quote('%"'.$opt.'"%').' OR '.'b.'.$db->quoteName($field->alias).' = '.$db->quote($opt).') AND ';