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.