I don't think i can truly fork HN. HN is not only the code, it's also pg's leadership and the community that has gathered around it. (if you can give me the code for that, i'd love to fork :P ) Anyone else than pg making this work is much more difficult, as HN already exists and critical mass will be hard to acheive.
The reason I think trolls won't post as much is that while trolling is based on impulse decisions, making a payment is a much more elaborate procedure and I don't think the impulse to troll would usually be enough to overcome that threshold. But even if it does, blocking that account, noting the payment details and blocking further re-registration with them raises the re-registration barrier even higher.
I think you underestimate the probability that the world does not divide neatly into trolls and non-trolls. Rather, the troll lurks within us all, and without constant vigilance, under the right conditions the troll will emerge!
In other words, users will not pay to troll. They'll pay to make a comment, which may or may not be a trollish comment. (A lot of trolls can't, in the heat of the moment, tell the difference between a troll and a comment -- that's what makes them prone to being trolls.) And then they'll have paid, which means there's no barrier at all to future trollish behavior from the same person.
That is certainly true. I've sometimes run across people cross-posting comments here and on Reddit who've actually taken the trouble to write two versions, a flame for Reddit and a civil version for HN.
Having done something similar, it's not necessarily taking the trouble to write two versions. Sometimes after writing one version, you mellow out, have a change of heart, or just realize you might not have been completly right (that "W" word :-), so the post on a complementary site is more thought out and polite.
Yes, I am talking about intended trolling. 'Accidental' trolls can be brought to shape by the community, as they always have. Also, the payment T&Cs should say that the community still has the right to kick them out, refunding the amount if necessary.
The reason I think trolls won't post as much is that while trolling is based on impulse decisions, making a payment is a much more elaborate procedure and I don't think the impulse to troll would usually be enough to overcome that threshold. But even if it does, blocking that account, noting the payment details and blocking further re-registration with them raises the re-registration barrier even higher.