If you want the rude and crude one, just make a new page that executes a db query:
select resid from reservations where end_date < mktime(0,0,0);
Then create a ResDB object
$db = new ResDB();
and loop through all reservation ids, calling
$db->delRes($id, null, false, 0);
I can't really provide all the code to create the page and DB connection, but you can copy that from the existing code.