I think the most obvious solution is more transparency as to how much individual campaigns, and individual campaign managers harm the forum. The forum could publish how many posts were deleted every week from people who are participating in individual campaigns (and in campaigns managed by specific campaign managers), the forum could publish how many users are banned (for insubstantial posts + paid signatures, and other reasons) in individual campaigns and in campaigns managed by individual campaign managers.

From the looks of it, the market is engaging in somewhat of self-regulation. For example, the fact that marcotheminer was entrusted by bit-x with 50BTC, and paid out participants with that 50BTC without incident would imply that many others would trust him to manage additional signature campaigns, however this largely has not been the case. On the other hand, carra23, who has essentially zero prior trading history, which would imply that she should not be trusted with large amounts of bitcoin, has been able to do (what I think more of less everyone would agree is) a good job of preventing low quality posters from entering her campaigns, and when low quality posters do get into her campaigns, she does a good job of managing the low quality posts, and as a result she has been able to generate a large amounts of additional business.