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.
For what its worth before hexbear disabled downvotes they looked at who had been systematically downvoting trans peoples posts and a couple transphobes got purged.
Also any drama is around downvoting, no cries about systematic upvoting. Seems like any drama can be avoided if downvoting is just disabled.
Vote manipulation using alt accounts also get dealt with: https://lemmy.ca/post/50545875?scrollToComments=true