Can any user downvote, or is some karma needed? It would be good if only users with karma at least, say, 20 could downvote, because that would prevent creating a new account for safe mass downvoting. (Similar system is used at StackExchange.) I'm saying this because if we adopt a policy of detecting and punishing mass downvoters, their logical next step would be to mass-downvote using a different account.

My opinion (but I have low confidence in my ability to correctly handle these situations) is the following:

If an obvious case of mass-downvoting is detected, there should be an ad-hoc tribunal made by three people from MIRI / CFAR / Trike. The tribunal should decide whether the situation deserves punishment or not. It is their choice whether their decision includes asking the offender's explanation. If the tribunal agrees that the situation deserves punishment, then:

The punishment should be public. A Discussion article describing what happened, who downvoted whom, and what is the punishment. Not a public debate about the punishment; only a public announcement of the final verdict. (The reason for this is that in my estimate most likely a member of some political faction was mass-downvoting a member of an opposing faction, and the public debate would bring too much attention to the factions; possibly suspicion or accusations that people are recommending more/less punishment because of their sympathies to one of the factions.)

If possible (if we have the necessary data), all votes made by the offender (both upvotes and downvotes, to anyone) during the last X months should be reverted. This is to say "we don't value your opinion". (Value of X is decided by tribunal, recommended value 3 or 6.)

If for technical reasons reverting recent downvotes is impossible, the victim should have restored 90% of the karma lost by mass downvoting to their account. (I say 90% because some downvoting is allowed.) Also, the same amount of karma should be removed from the offender's account.

Optionally (depending on tribunal's decision) the offender could be banned. The rule of thumb is that if it happened first time, and was only against one person, banning is not necessary; repeated offense or mass-downvoting of many people deserves banning.

Summary: Mass downvoting should be punished publicly, karma restored, repeated offences should lead to ban. The details should be decided by an ad-hoc tribunal of site owners/moderators, not by a community debate.