They are quite capable of removing all his karma, or even setting it very negative, which would remove his ability to downvote. To the extent there are technical problems, they could give him a temporary ban while these are worked out.
Perma-ban is a deliberate choice by the moderator(s), not something they were forced into.
Below some negative karma threshold, there is a forced delay between comments (after posting a comment you have to wait a few minutes before you can post again).
As the topic seems to imply: they don’t actually seem to have a way to keep him from using his karma.
They are quite capable of removing all his karma, or even setting it very negative, which would remove his ability to downvote. To the extent there are technical problems, they could give him a temporary ban while these are worked out.
Perma-ban is a deliberate choice by the moderator(s), not something they were forced into.
By the way, what are the consequences of low karma? Not being able to downvote, not being able to post articles… anything else?
Below some negative karma threshold, there is a forced delay between comments (after posting a comment you have to wait a few minutes before you can post again).