Actually, I was going to recommend against a detailed formal policy. Codified rules seems like an invitation for passionate arguments about Stuff That Doesn't Matter™. If we need a rule, how about:

If you are acting in a way incongruent with the greater harmony of LessWrong, a moderator may private message to ask you to stop. If you don't stop, you may be banned.

Also have a stated social norm:

Don't downvote comments because of the author. There's no hard and fast rule, but if you've downvoted someone more than 8 times in one day or read through someone's comment history and downvoted past the first page, you are doing something wrong.

In the specific case of mass downvoting, if a moderator gets a complaint I think the typical procedure should be:

1. Moderator private messages the alleged downvoter a non-accusatory FYI letting them know that someone felt they were heavy on the thumbs-down and it might be a good idea to take a breather
2. If vindictive downvoting continues, look into the logs, do some quantitative analysis, and evaluate. Most probable conclusion is that the downvoter isn't being vindictive. But if, regrettably, the downvoter appears to have a bullying voting pattern, the moderator should private message them "Really, you should stop. Even if there are good reasons for all of your downvotes, you shouldn't continue just because it looks bad. There are plenty of other people are around who can pick up the slack if you bow out of downvoting."
3. If abusive downvoting continues, give a final warning, then ban.

But before you complain to a moderator: Please consider the possibility that your comments are just bad, that someone might disagree with you for legitimate reasons, and that negative feedback is information positive.