SO also has direct rate limiting (40 votes a day). I do think that it makes sense to have a separate rate limit for user-user links; maybe I can vote 100 times a day and have it be normal, but voting even 10 times a day on a particular user might means something funny is going on.
Yes, it would be reasonable to have separate per-user/user link limits. Though the limits could be a function of the number of comments that user made recently -- if someone gets into a manic mode and posts dozens of trash comments in a few hours...
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?