Being able to put a warning at the top of those threads saying "This post is complete nonsense" would not necessarily be a bad thing. Having said that, given how petty some of the endless squabbles on the reputation board are, even among some of the most senior members on the forum, there is absolutely zero chance that such a feature wouldn't be misused.
That's almost censorship via influencing users opinions before they've even read the thread though. I hate the idea of a demerit button also, but I can understand why users would want one. Even if this was implemented perfectly it would cause a lot of hassle, but I don't think because it would cause a lot of arguments between users that it should be a deterrent of adding something to the forum. Usually, not always you try to do something for the greater benefit of the forum, but how much value would a demerit button provide to the community as a whole? Honestly, we would likely only be providing a little satisfaction towards the person who wants it de-merited rather than providing something of use to the community. As for the candidates that wouldn't misuse a demerit, I actually personally think a lot of merit sources would be unsuitable for the job, and for those that are suggesting DefaultTrust have a say in it, then I would completely disagree with that. Merit sources have been selected for their ability to reward posts which have been deemed high enough quality by themselves, and their merit behaviour has been deemed at least satisfactory by theymos on review.To expand on the unsuitability of merit sources for demeriting, I actually think there's only a select few on this forum that wouldn't intentionally misuse it, and then might accidentally from time to time. Its one thing meriting posts that you like, but we are human, and no matter how much there will always be bias among users, and no one is perfect that's for sure. We've seen the arguments between DefaultTrust, and we've seen the disagreements of certain merit source behaviour. The unfortunate truth is demerit could be a powerful tool.
Here's an example of demerit misuse, lets say that demerit was allowed to be used by anyone that has earned x amount of merit. Well, lets say that user wants to be in a signature campaign, and there's a minimum merit requirement, and the campaign is currently full. The malicious user could prawl through the participants, and go through their entire history, and find low quality posts, because lets admit we've all got low quality posts at some point in time. They could demerit these posts, and then lower that users merit enough for them to be potentially removed from the signature campaign. I understand this is a extreme example, and is unlikely to be done without being caught, but this can be applied to almost anything. The thing is, people are competitive by nature, and there's several statistics out there documenting users merit gain, and I can guarantee you competitiveness will influence certain users judgement, and cause them to demerit try to "beat" that users merit score.
These are just examples, and I'm probably missing another 100 things demerit could be used for with malicious intent. Its a can of worms, and if this was to ever be implemented those that can demerit need to be selected carefully, and there should probably only be a handful of them rather than the amount of merit sources we have.
I like merit. You find a post that you subjectively think is good quality, and you reward it which has some benefits possibly to the user that posted it. If you don't like it, you don't merit it, and it has no negative effect on the user that posted. If demerit was a thing, users could subjectively view these statistic threads (as an example) that are posted fairly regularly now as bad posts, and demerit it into oblivion. Although, some of the statistic threads may be a little tedious, they are usually of good quality.
Having said all of this, I do share Gmaxwell's, and others discontent about users getting away with such bad quality posts, and sometimes just purposely spreading misinformation, but I think its better to counter that within a post of your own, than potentially implement a system which could be misused outside of these poor quality posts that have been specifically mentioned.