To my knowledge there is nothing stopping people from answering old RMT posts, so long as we approve them in the admin. If memory serves, there is extra moderation on Meta/RMT for old posts, but if there is not I agree there should be. Maybe not for everyone though, I think there should always be a way to bypass the admin if you are experienced.
Might also mention here, I'm no longer as confident that 1 would be the best number. All it would take is for one person to write an exploit then distribute it online and the system is broken. I still don't think anyone would bother, but if we want to remove all chance of spam possibly occurring, 2-3 is best since it would be hard to make multiple posts to pass the system without us becoming suspicious. I'll edit my post to reflect this. Thanks again for the input guys, you swayed me :)