I don't read Error's proposal as being about spiting him, but about deterring his antisocial activities. (But I agree that Error's specific proposal goes too far. Reversed stupidity is not intelligence, etc.)

as a one-off action, or as a threat alone, it might be viable as a deterrent.

Yeah, this is one of the points where I agree with Lumifer. I'm one of Eugine Neir's biggest targets, as is gjm, but I think reversing karma is not the best move. Negating the karma would be optimal here.

You could perhaps nullify the upvotes after a few months to preserve the system in the long run. The idea is that the short-term effect of his actions should be net-negative from his own perspective.

Other ways of achieving the same effect may also work; that was just what I came up with after five minutes of thinking about it.