I'm not a huge fan of The Awful Forums, but I do like their probation system. Right now, a moderator on HN can either permanently take away a user's privileges or do nothing. Probation is a nice middle ground. If the user persists in behaving badly, then the banhammer can be brought out.
It's also useful to have a notice such as, "This user's commenting privileges were suspended for 100 hours for this comment." By making the punishment public, the commenter is (hopefully) shamed into behaving better in the future. And like you said, it may even prevent others from behaving similarly.
This is also why I liked that comment points used to be displayed -- you could learn what a good comment was by looking for the ones with zillions of points, and also what a bad comment was by looking for the ones that sat at 1-2 points while responses to it scored much higher.
> "Isn't it enough that downvoted comments become gray?"
Nope. That doesn't show you the difference between a barely-passable comment and an excellent one.
I learned a lot during my early HN days, when comment scores were visible, simply by looking at what sort of comments tended to score 5 points vs 1 point, or 15 points vs 5 points, or 40 points vs 15 points. Here on HN, "just post short memes" tends to be met with negative signals; thoughtful, helpful, and accurate responses are met with positive signals. I found it helped to be able to see that, and I think many newer HN commenters have the same misconception as you do because they haven't had that same training.