The mod banning these users is the same mod who made the posts they downvoted. This is mod abuse, turning the downvote button into an auto-self-ban button.
The message is “If you disagree with me, you will be banned”
Monitoring and banning users for using lemmy as intended to signal boost your opinion should be grounds to have all mod privileges removed. This behaviour undermines the integrity of the server and the wider fediverse.
If your posts turn up in /c/all they’re going to get treated accordingly.
And this is fine. /c/all should let users downvote posts they don’t like so popular stuff can rise to the top. That’s what makes /c/all sometimes worth looking at.
Otherwise, it’ll just fill up with all sorts of crap from communities with no downvoting rules, including edgy borderline racist stuff that’s not quite bad enough to get banned, or just shitty positivity memes copied from somewhere else.
Your problem is that you can’t delist your community from /c/all. That sucks, but right now your posts are turning up in two different communities with different expectations and you just need to deal with that.
Your admin can, that’s a more effective way to deal with that than downvoting