I've encountered a few of those users, and personally I ban them.
My criteria:
- They must be downvoting a majority of posts, where it's clear there is no real discrimination on quality (as then some posted close together would be spared)
- I notice it happening across multiple communities/instances
- I see the same name again and again, day after day
- I never see that user upvote anything anywhere
I know there was one user who was mass downvoting to, according to them, mark posts as read.
I thought that was fairly ridulous, as why not just mass upvote instead to achieve the same effect? Wasn't sure I fully bought that, as they were dismissive when someone asked them to stop.
Lemmy is still small, and it seems unwise to allow mass downvoting to potentially discourage or limit the reach of people putting in the effort to help this place grow.
I've never had a downvoter message me to ask why they were banned or dispute it, so I figure they didn't even notice that they no longer saw the stuff that caused them to downvote in the first place.
Why not just sort by Hot, New, or Scaled instead? The popular posts in Active sort will naturally go away as discussion in them dies down (comments boost posts too).