Why don't you lock all threads which are 6 months and older? This will prevent new users especially, posting in them and saving you a few seconds by not having to lock, delete or start new threads.
But with some if not all other forum softwares, you can mass lock, move, merge, delete etc. via an admin control panel. I'm surpassed this vBulletin hasn't got this option.
What now will take days or weeks to do could have been achieved within minutes.