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.
So challenging that you need to protect your own feelings by banning anyone who downvotes you, so that you don’t decide to leave a community that you’re already in a position of power over? Grow a pair.
What “power” does the mod have if the community doesn’t have other members?