From your comments I take it that you don't think there is anything else that can be considered (aside from the merit system) that would increase the quality of the discussions? It would be disappointing if the quality issue is just a fact of life which simply has to be accepted.
There probably are ways to go about dealing with the spam, but I don't think this can be done without restrictions which affect legitimate users. At the moment we've only stopped users who post bad quality posts from ranking up not removing it. Generally though I think the forum is in a much better place than it was a year or two ago.
There's a constant juggling act when implementing restrictions and dealing with the spam. If there was a solution which would couldn't be abused and wouldn't limit legitimate users that would be great and would probably be implemented in a heart beat.
Yes, there are always two sides to things.
So, how about the idea of write-restricted areas that are accessible (for posting) only by invitation or, as was suggested above by Jet Cash, only for accounts with a certain rank?
I'd like to suggest a third alternative to use the number of merits received, instead of the rank / total number of merits. I've seen posts from hero and legendary members that don't pass muster, and those individuals have actually not received any merits, aside from the initial allocation. Although I have no idea how much effort this would take to implement, I can't imaging it would be too difficult. And since awarded merits are to a large extent (although not completely) reflective of the quality of the contributions, it seems like an appropriate yardstick.
And using the awarded merits as an entry point to the write-restricted areas would not materially affect legitimate users. Since users can still read the forums, they are not loosing out on the content. But only those who have demonstrated in the open areas that they can provide valuable input and have been recognized as doing so by having received merits, can start contributing actively to those write-restricted areas. Whether this applies to sections in the forum, or at the thread level is perhaps a secondary question, but I can't see how this would not lift the quality, while not being too restrictive either.
Any thoughts?