I saw an interesting feature on another forum that I think would be really useful here. It was on the amazon.com customer discussion forum.
At the bottom of every post there is the following question, "Do you think this post adds to the discussion?" There are two buttons to click, yes or no. If there are a substantial number of no votes, then it shows up as if the user was in an ignore list. i.e. you don't see it by default, but you can see it if you really want to. You have to be logged in to vote, and it takes a few votes for it to dissapear. For an example, see
here.
I think that this would complement the existing features of this forum.
James.