I am surprised that access to his account was not simply removed.

I checked my recent comments, because I was planning to say that so far he doesn't seem to have offended after the banning, however I found no evidence of that. The two comments from the last 2 days, which I had noticed did't get any downvotes initially, now had one (as do my other comments). This doesn't prove much, so I checked all the other users who were getting mass downvoted for extended periods (the list in the post) and it seems like all their recent comments had at least 1 downvote as well.

The above is not definite proof, but what I suspect might be happening is that Eugine's bot/script is still running, despite him leaving the site.