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Maybe you should be focusing more effort on users like this one, who has been posing nothing but abuse for a monthThat account has been banned since the day it was created. Its comments have never been anything but [dead]. That's the maximum penalty that exists on HN. Several other accounts related to that one are also banned. This is standard.
(For readers who don't have "showdead" turned on in their profile: the account anigbrowl mentioned has been posting things like "Shut up, $SLUR", "Fuck off, $SLUR", and worse. They are banned and all their comments are killed automatically. This means that no one sees them unless they turn the 'showdead' setting on in their profile. Any user is welcome to turn that setting on, but please don't forget that if you do, you're signing up to see the worst that the internet has to offer on HN.)
Banned accounts can continue to post, but their comments are killed by default. Why? Because otherwise they'd just create new accounts and pick up where they left off. Since new accounts are unbanned at first, there would be a time lag before we could catch those and ban them again. In the meantime, more abusive posts would get through.
In other words, allowing banned accounts to post (but making their posts default-invisible) is the way to minimize their effect on a large open forum like HN, where there's no barrier to creating accounts. Attempting to restrict them further would just end up exposing more people to the abusive posts.
HN would have ceased to exist long ago if we were "doing nothing about overtly abusive behavior", so I was a little taken aback at your assumption there.
I agree, of course, that 13_9_7_7_5_18's posts were worse than computerthings's. That's why we banned the former immediately, and the latter only after warning them (many warnings, actually, if you count previous accounts that we banned) and only after they'd broken the site guidelines many times.