Either alter the setting in admincp.php or comment out/delete this from your include/cleanup.php file
PHP Code:
// delete old torrents
$dt = sqlesc(get_date_time(gmtime() - $torrent_ttl));
$res = mysql_query("SELECT id, name FROM torrents WHERE added < $dt AND seeders=0 AND leechers=0 ");
while ($arr = mysql_fetch_assoc($res))
{
@unlink("$torrent_dir/$arr[id].torrent");
mysql_query("DELETE FROM torrents WHERE id=$arr[id]");
mysql_query("DELETE FROM peers WHERE torrent=$arr[id]");
mysql_query("DELETE FROM comments WHERE torrent=$arr[id]");
mysql_query("DELETE FROM files WHERE torrent=$arr[id]");
write_log("Torrent $arr[id] ($arr[name]) was deleted by system (older than $days days)");
}