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First of all, the point of deleting a thread is that it is removed from the forums and can no longer be accessed. What you are suggesting defeats the purpose.
Second, I would be all for a "lock" function but this is not Vbulletin and every change made needs to be coded by a dude at the NS office. There is a discussion right now in the mod forum about ways to improve the site, with the caveat being "no new programming" because the stuff that is already lined up to be coded is going to take months.