Downvotes on posts/comments older than X time affect the downvoters karma the same way as they do the downvoted.
Downvotes made after X time from the original posting affect karma at a rate of y%
Downvotes from users with karma below X don't affect the downvoted's karma score
All of these are made on the assumption that malicious downvoters are engaged in a E-Peen measuring contest using Karma as the measuring tool.
I've gotten sufficient evidence from support that voiceofra has been doing retributive downvoting. I've banned them without prior notice because I'm not giving them more chances to downvote.
I'm thinking of something like not letting anyone give more than 5 downvotes/week for content which is more than a month old. The numbers and the time period are tentative-- this isn't my ideal rule. This is probably technically possible. However, my impression is that highly specific rules like that are an invitation to gaming the rules.
I would rather just make spiteful down-voting impossible (or maybe make it expensive) rather than trying to find out who's doing it. Admittedly, putting up barriers to downvoting for past comments doesn't solve the problem of people who down-vote everything, but at least people who downvote current material are easier to notice.
Any thoughts about technical solutions to excessive down-voting of past material?