As previously discussed, on June 6th I received a message from jackk, a Trike Admin. He reported that the user Jiro had asked Trike to carry out an investigation to the retributive downvoting that Jiro had been subjected to. The investigation revealed that the user Eugine_Nier had downvoted over half of Jiro's comments, amounting to hundreds of downvotes.
I asked the community's guidance on dealing with the issue, and while the matter was being discussed, I also reviewed previous discussions about mass downvoting and looked for other people who mentioned being the victims of it. I asked Jack to compile reports on several other users who mentioned having been mass-downvoted, and it turned out that Eugine was also overwhelmingly the biggest downvoter of users David_Gerard, daenarys, falenas108, ialdabaoth, shminux, and Tenoke. As this discussion was going on, it turned out that user Ander had also been targeted by Eugine.
I sent two messages to Eugine, requesting an explanation. I received a response today. Eugine admitted his guilt, expressing the opinion that LW's karma system was failing to carry out its purpose of keeping out weak material and that he was engaged in a "weeding" of users who he did not think displayed sufficient rationality.
Needless to say, it is not the place of individual users to unilaterally decide that someone else should be "weeded" out of the community. The Less Wrong content deletion policy contains this clause:
Harrassment of individual users.
If we determine that you're e.g. following a particular user around and leaving insulting comments to them, we reserve the right to delete those comments. (This has happened extremely rarely.)
Although the wording does not explicitly mention downvoting, harassment by downvoting is still harassment. Several users have indicated that they have experienced considerable emotional anguish from the harassment, and have in some cases been discouraged from using Less Wrong at all. This is not a desirable state of affairs, to say the least.
I was originally given my moderator powers on a rather ad-hoc basis, with someone awarding mod privileges to the ten users with the highest karma at the time. The original purpose for that appointment was just to delete spam. Nonetheless, since retributive downvoting has been a clear problem for the community, I asked the community for guidance on dealing with the issue. The rough consensus of the responses seemed to authorize me to deal with the problem as I deemed appropriate.
The fact that Eugine remained quiet about his guilt until directly confronted with the evidence, despite several public discussions of the issue, is indicative of him realizing that he was breaking prevailing social norms. Eugine's actions have worsened the atmosphere of this site, and that atmosphere will remain troubled for as long as he is allowed to remain here.
Therefore, I now announce that Eugine_Nier is permanently banned from posting on LessWrong. This decision is final and will not be changed in response to possible follow-up objections.
Unfortunately, it looks like while a ban prevents posting, it does not actually block a user from casting votes. I have asked jackk to look into the matter and find a way to actually stop the downvoting. Jack indicated earlier on that it would be technically straightforward to apply a negative karma modifier to Eugine's account, and wiping out Eugine's karma balance would prevent him from casting future downvotes. Whatever the easiest solution is, it will be applied as soon as possible.
EDIT 24 July 2014: Banned users are now prohibited from voting.
First of all: I agree with shminux. If the point of the site is to create a community that can produce high-quality discussion, rather than one that's full of cooperatebots, banning users who make quality contributions for defecting from social norms that don't directly involve their contributions seems contrary to that point. Maximizing discussion quality requires tradeoffs: if the presence of someone who produces high-quality discussion is so opposed by others who produce same that the town isn't big enough for both of them, someone has to go. But was this the case here? -- that is, what would be the effect of Eugine_Nier's mere presence, as opposed to his ability to mass-downvote? The town isn't big enough for both Eugine_Nier's downvoting and its targets / others who see the community as having become less cooperatish due to that downvoting.
This also sets a dangerous precedent, one which I've seen play out before: the most 'elitist' (offensive/annoying to certain other members, usually of lower quality) users get banned for being 'elitist'/offensive/annoying, with no regard to the effect on quality -- so, naturally, quality goes downward. If you don't bite the noobs, you get Eternal Septembered -- but if you bite too hard, you drive off other users who make high-quality contributions. It's often hard to see which way the tradeoff should go -- on the forum where I most recently saw this dynamic play out to its Septembered conclusion, there were six bans, I only disagreed with one of them, and the most that any user I know of disagreed with was three of the six -- so my intention in pointing this out is not to suggest any hard-and-fast rule, but to call to attention the existence of this tradeoff and suggest an optimization criterion for moderation policy going forward.
I don't know what the LW consensus is on the issue of quality vs. quantity; I naturally favor quality, but I don't have any financial interest, direct or indirect, in any of the organizations/charities in the same general area as LW, so those who are affiliated with MIRI or similar, or who place a much higher priority on their ability to get as much money as possible, may have different interests. I would suggest, however, that quality has a quantity of its own -- that is, that optimizing for high-quality discussion/contributions/etc. will serve as a draw to people who want such an environment, since there aren't all that many environments for that -- and furthermore, that the people drawn in by that are more likely to be well-placed with regard to the ability to propagate the rationalist memeplex and the [concomitant?] awareness of the aforementioned organizations.
Now, another interesting question is: given that the karma system is open by its nature to abuses of this sort, what ought to be done about it? Downvote-bombing is clearly harmful; how can the potential for it be reduced/eliminated?
A few possibilities that I can think of:
Making it so that you can't downvote more than ten posts by one user in a day. (But this might just make it harder to notice.)
Automatic alerts to moderators if a user downvotes more than fifty posts by one user in a week.
A 'tactical nuke' option, to remove all karma from a user found to practice mass downvoting.
A karma lockout option, to remove a user found to practice mass downvoting from the karma system entirely: their posts would still be able to accumulate karma, but they wouldn't be able to either upvote or downvote posts.
As above, but only blocking downvotes.
Simply knowing that you would be outed is likely to be enough of a deterrent, no need for advanced technical solutions.