[Andreas]

@Indyshooter
Try this:

1. [sql]ALTER TABLE forum ADD pruneafter INT DEFAULT 0 NOT NULL[/sql]
2. In admincp/forum.php FIND
   
   PHP Code:

   print_yes_no_row($vbphrase['warn_administrators'], 'options[warnall]', $forum['warnall']); 

   BELOW that ADD
   
   PHP Code:

   print_input_row("Prune After<dfn>Stetting this to a value > 0 will automatically prune all threads from this forum where the last reply is older then the set amount of days</dfn>", 'forum[pruneafter]', $forum['pruneafter']); 

3. Place the following as prune.php in includes/cron
   
   PHP Code:

   <?php
error_reporting(E_ALL & ~E_NOTICE);

if ($DB_site == NULL)
{
    exit;
}

require_once('./includes/functions_databuild.php');
$forums = $DB_site->query("SELECT forumid, pruneafter FROM " . TABLE_PREFIX . "forum WHERE pruneafter > 0");
while ($forum = $DB_site->fetch_array($forums)) {
  $threads = $DB_site->query("SELECT threadid FROM " . TABLE_PREFIX . "thread WHERE forumid=$forum[forumid] AND lastpost <= " . (TIMENOW - ($forum['pruneafter'] * 86400)));
  while ($thread = $DB_site->fetch_array($threads)) {
    delete_thread($thread['threadid'], 0);
  }
  build_forum_counters($forum['forumid']);
}

log_cron_action('Forums pruned', $nextitem);

?>
4. Go to Forum Manager, Edit the Forums you want to be pruned automatically and set the number of days
5. Create a Cron Job for prune.php

Use AS-IS, no warranties.