What admins (or for that matter automated software) could do is to publish the information. This can be done without determining motives. I'm thinking, e.g., that if user A mass-downvotes user B (according to some heuristic whose details are important but I'm not going to try to nail down here) then (1) when you go to the user page of A or B, that information is listed near their karma score, and maybe (2) when you hover over user B's karma score on any page, it indicates the fact. ("Mass-downvoted by Dias, gjm, and 6 others" or something.)
In that case, it would still be possible to downvote everything a user posts, if you truly think everything they've done is bad. You'd just have to accept that in that case your decision to do so would be publicly visible. In cases where someone really just is posting a lot of stupid stuff, this would probably enhance your reputation among sensible LW users, rather than hurting it.
(Roughly what might that heuristic look like? Something like this: "A has mass-downvoted B if A has downvoted at least 20 of B's contributions, has downvoted at least twice as many of B's contributions as s/he's upvoted, and there's some sequence of at least 20 consecutive contributions from B of which A has downvoted at least half." Every single detail of that is nitpickable; I'm just gesturing vaguely towards the sort of thing that might work well.)
Every single detail of that is nitpickable; I'm just gesturing vaguely towards the sort of thing that might work well.
That might actually be a devil-in-the-details thing, and gameable by rules mechanics, too.
Another point is technical issues -- you want to keep track of interactions (up/downvoting) between pairs of users and that's an O(n^2) problem.
I'm just tired of the signal pollution, and would like to be able to use karma to honestly appraise the worth of my articles and posts, without seeing 80% of my downvotes come in chunks that correspond precisely to how many posts I've made since the last massive downvote spree.
EDIT to add data points:
Spurious downvoting stopped soon after I named a particular individual (not ALL downvoting stopped, but the downvotes I got all seemed on-the-level.)
One block of potentially spurious downvoting occurred approximately one week ago, but then karma patterns returned to expected levels. I consider this block dubious, because it reasonably matches what I'd expect to see if someone noticed several of my posts together and disagreed with all of them, and did not match the usual pattern of starting with the earliest or latest post that I had made and downvoting everything (it downvoted all posts in a few threads, but not in other threads), so I'm just adding for completeness.
Spurious, indiscriminate downvoting started up again approximately half an hour ago on Sunday (12/1/2013), around noon MDT.
Edit: And now on Tuesday, 12/3/2013, at 10 AM, I'm watching my karma go down again... about 30 points so far.
Edit: And now on Saturday, 12/14/2013, at 2 PM, I'm watching my karma go down again... about 15 points so far, at a rate of about 1-2 points per second.