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The more I think about it, the more I like a shadow-ban type of system. The question is, how to implement it properly so it doesn't prevent people from wanting to register on Bitcointalk and only keeps the bad guys away? We know how resourceful they can be when they smell blood and an opportunity to scam and abuse. They can easily pretend to be regular newbies wanting to before part of the community, only to show their real faces once their shadow-bans are lifted.
Its hard to predict how much it would be effective in its initial state, but it would definitely help. I would say, that a user doesn't get whitelisted from just one post, I think having a undefined amount of posts before the shadow ban was lifted would probably be of benefit here. Although, that would likely be very similar to the newbie jail, and probably more restrictive.
Unfortunately, for those that want to put the effort in, they would be able to get around most restrictions we put in place, and that isn't something we could combat effectively. However, as we know many of those that are spamming the forums, aren't particularly fussed on making any effort at all, even when it means they can earn money from it. So, while its quite possible to cheat your way around the system, it would still probably prevent a lot of the spam getting out.
So while it isn't a catch all, it would certainly contribute to the quality of the forum, and would almost certainly prevent bots from being exposed to the public.