[Delphy]

I wrote something similar for my board a while back.

What I would do is change the query to:

    $DB_site->query("UPDATE " . TABLE_PREFIX . "thread SET open=0 WHERE forumid IN(". FORUM_IDS .") AND open <> 0 AND FROM_UNIXTIME(lastpost) <  DATE_SUB(CURDATE(),INTERVAL ". TIME_SPAN .")");

This way you are:
1) Only locking threads that aren't already locked (saves re-locking already locked threads)
2) Basing it on the lastpost in the thread, so you don't lock active threads that are created within your timespan

What I also do is hard delete any that are locked and within another timeframe using the same cron job.

Regards,
Delphy