As one of those targeted, I thought about what I would change if I could. All I came up with is posting mass downvoting stats periodically. If people knew their actions would be detected and made public, they would probably refrain from doing it in the first place.

I am not familiar with the LW database schema, but It is probably trivial to write a SELECT statement which finds users who have been downvoted more than, say, 100 times in the last month, and find the most prolific downvoter of that user. Hopefully this can be a roughly O(n) task, so that the server is not overloaded. I'm sure Jack can come up with something sensible.