Thanks for the explanation of the shortcomings of decentralized forums. I've always been curious why they haven't blown up and this seems like the actual reason. Could create a market opportunity if someone could address the issues Theymos pointed out.
The question is, how. It would be impossible to remove things such as bots, spam or other bad things in a decentralized forum as there would (by concept) be no moderation. It would rely on responsibility by the posters on the forum. The internet isn't responsible.
I am not suggesting bitcointalk do anything but hypothetically speaking maybe a market based solution like requiring some kind of micropayments to post or participate (see email micropayments concept [http://www.coindesk.com/new-email-service-fights-spam-bitcoin-micropayments/]).