I've banned them without prior notice because I'm not giving them more chances to downvote.
I think a "We've observed X. It appears to be bad behavior. Do you have an alternative explanation?" discussion should be started in any case. Otherwise there will be no justice for false positives.
Is the "because I'm not giving them more chances to downvote" a real argument? It won't be if it's technically possible to prohibit downvoting (maybe by temporarily taking away their Karma, so that the Karma-based voting limits would kick in), or if it's possible to eventually retract their (recent) votes, so that current votes won't matter as much.
I think a "We've observed X. It appears to be bad behavior. Do you have an alternative explanation?" discussion should be started in any case.
I don't think you get effective forum moderation by having public discussions about every moderation action.
Are you aware of a functioning online community which does things like that?
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?