Chaosmosis has had a significant number of upvoted comments. Some of his conduct has been very obnoxious and counterproductive, but I don't think he's reached a point where it's reasonable to write him off as unable to learn from his mistakes. At the least, I think his continued presence is more likely to be fruitful than a couple of other recently active members whose contributions have been uniformly downvoted.

Point of clarification: does banning a user on LW do anything but force them to create a new user account if they wish to keep contributing?

I have been using Wei_Dai's awesome greasemonkey script for a while now to filter out some of the users I find valueless, so having them create multiple usernames to dodge the banhammer would be a mild nuisance for me.

So if that's all it does, I'm somewhat opposed to it, but willing to remain neutral for the sake of not inconveniencing other people who don't use that script for whatever reason.

OTOH, the responses to those users are themselves mildly annoying, so if the banhammer does something more worthwhile than that, then I might be in favor of it.

Is there a certain threshold that once passed, personal karma totals no longer matter?

I am convinced that you are wrong in point of fact. I am telling you this so you can adjust your feelings to fit reality. As long as you feel Alicorn or others are out to get you, or profit by hurting you, you will probably not be able to make a useful contribution to discussions or enjoy them yourself.

From my experience as a longtime reader of the site I can tell you that reputation on LW is not normally gained by attacking anyone, even if everyone else agrees in disliking the target or their comments. We have community values of responding to the factual content of each comment, with clear literal meaning, and without covert signalling. Reputation is gained by contributing to the conversation.

We also require civility, and since people are often bad at predicting how others will react to borderline comments like "dumb", it's best to be what may seem to you to be extra-civil just to avoid conversational traps. You have edited the comment that seemed to start this whole subthread, and I haven't seen the original, so can't comment more specifically.