Would it be problematic to put a blanket ban on upvotes and downvotes of posts that are older than 30 days? Changes in karma to old posts are no longer an especially useful signal to their author anyway. Such a ban could be a cheap way to mitigate downvote stalking without significantly impacting current discussions.

An attacker could still use multiple accounts to mass-downvote everything from a user in the past 30 days. On the other hand, it's possible that some users' comments were uniformly bad. For the purpose of providing a useful signal, I think we only need enough downvotes to go just a bit negative. People respond disproportionately strongly to loss than to gain, after all! The karma of a particular comment could be capped at no worse than, say, -3, regardless of how many downvotes it received. That would be a cheap way to reduce the possibility of malicious mass-downvoting.