There is currently no such thing as a "demerit". I'm hoping that the positive merits alone will be fine. I could add demerits pretty easily later on if necessary, though.
So looking at this quote theymos may consider implementing a demerit system in the future. I'm largely against the idea as I think there's too much room for abuse and this is backed up by the system that Reddit currently uses.
I would like to add though that the merit system has really introduced a new dynamic to the forum and I think it's working as intended and will likely get better as more merit sources are added. But it would be interesting to see what the community as a whole feels about the possibility of adding this to the forum and hopefully if the current system does fail for what ever reason we can come with another solution instead of adding a demerit system.
Here's a few thoughts of mine:
1. Moderation by the community
If a post goes below 0 and into the negative it could automatically be hidden and have a collapsible menu to show the reply if anyone is curious what it says. This way spam posts or blatant lies etc could be hidden by default and make the forum more readable. This would also reduce the workload on the staff.
This also has it's drawbacks as this could very quickly be abused or misunderstood as I think it is on Reddit. Posts which someone doesn't agree with will likely be down voted even though they might be constructive and add to the discussion. Even if demerits were only given to certain members such as staff/merit sources these people would also likely be subjected to this sort of behavior.
2. Who can demerit and how they are generated
It's worth considering only giving merit sources the ability to demerit posts. They are hand picked personally by theymos and are probably trusted by him to distribute merit fairly. There's still issues where if someone has a personal grudge against someone they could use this feature maliciously I don't think anyone is perfect and might be tempted to use it for malicious purposes. If a demerit is implemented existing merit sources would likely have to be reviewed again to make sure that they understand that a demerit isn't something you give because you don't agree with the purpose but is there to identify poor quality posts.
It's also worth thinking about how they would be generated especially if it's not limited to merit sources. Merit sources would likely not need a limit to how many posts they could demerit but if anyone can use it on the forum then there needs to be a fair way to earn it. I think just giving certain people or merit sources the ability to demerit is probably the best choice.
3. Preventing members getting comfortable and continuing their old ways
Once a member has reached their desired rank they could then revert back to their old ways and start spamming nonsense again. There's currently no punish for someone who's not breaking the rules but posting garbage. A demerit system would likely prevent this and would give the potential to derank members. Too prevent abuse and inconsistencies a demerit would only allowed to be sent once per post from the same person. To prevent someone thinking "This user doesn't deserve this rank" and gives them like 20 demerit points.