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.
This is why it would be good to limit downvoting to subscribers only
It should absolutely be an option (it is on Piefed) - not mandatory, but anyone could subscribe to downvote anyway - and doing so would also in itself be harmful for small communities trying to gain new users as they wouldn’t have enough subscribers to upvote content posted on the community.