Do I understand it correctly that the behavior you describe is "downvote every new comment from user X when it appears" (as opposed to "go to user X's history and downvote a lot of their old comments at the same time")?
Because when hearing about karma assassinations, I always automatically assumed the latter form; only the words "early downvote" in Nancy's comment made me realize the former form is also possible.
A possible technical fix could be to not display the user comment's karma until at least three votes were made or at least one day has passed.
Also, off-topic: Crocker's Rules seem to be popular in out culture; maybe it would be nice to integrate them into LW user interface. For example user could add their "anonymous feedback URL" in preferences, and a new icon "Reply Anonymously" would then be displayed below all user's comments and articles.
Crocker's Rules
Crocker's Rules aren't about anonymity.
Theoretically it might be useful for people to be able to set a visible flag "Talk to me under Crocker's Rules" -- but I suspect that it will immediately degenerate into a status sign.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.