$sql = "SELECT user_id FROM ". BB_BT_DLSTATUS ." WHERE topic_id = $topic_id AND user_id != {$userdata['user_id']}";
$sql = "SELECT user_id FROM ". BB_BT_DLSTATUS ." WHERE topic_id = $topic_id AND user_id NOT IN ({$userdata['user_id']}, ". EXCLUDED_USERS_CSV .")";
Lange, читать чужие сообщения не есть хорошо.