As I understand it and have observed, users are shadowbanned because they've shown that they sometimes act in ways that are detrimental to the community as a whole. By continuing to act that way, they've accepted that they might be banned. Users can also appeal the ban by contacting the mods by email.
I leave showdead on and have vouched for comments of banned users. I've also seen users continue to spew vitriol after they know they are banned. Is the type of user that would pointlessly continue to contribute negatively in this fashion have on balance a positive impact on the community? Arguing that they're behaving this way because they've been banned shows that they don't have the maturity to recognize that their behavior continues to reflect badly on them. It's not going to change the fact that they're banned.
You're right that if you're a determined troll, you can get around a shadowbox. From what I've seen, banning has been on balance effective to deal with uncivil behavior. Creating a new account does add some cost to the ban. Relying only on down votes puts the cost entirely on the community at large at no cost to the user behaving badly. Do you think it's fair to the community to bear the cost of the user's behavior?
Do you have any other ideas on how to effectively shape community behavior, preferably ones that move the cost from the community to the user behaving badly? I'm honestly interested, because I've seen enough complaints about banning that I'd like to hear about alternatives that would considered fair and still be effective.
It would be great if everyone would contribute in a positive, or at least civil, way. In larger online communities it's much easier for users who contribute negatively to destroy a community where positive users no longer want to participate. Each community has its own standards and cultures (and what's considered good and bad behavior), which is great! So far, I think it's pretty remarkable that HN has been able to maintain the community it has in comparison to other online communities of similar users.