sorry for typing this up with so many line breaks but it just helps organize my thoughts… such imageboard software should also be able to cope well with VPN and Tor users. if you need an account to post anyway, and all messages from an account can be easily removed, then you don't have to worry about VPN or Tor users circumventing bans and stuff like that. The downside is people need accounts, but to keep it as anyonymous as possible, you should be allowed to create/request an account from a VPN or Tor, and it should have as little information as possible. I think just an ID number and passphrase is all that's needed. Maybe even just a long unique key.
If this kind of software were available, you could start your own board and not really have to worry about moderation as much. Only accept as many new accounts as you and your janny staff can handle, if you even have jannies or mods. One guy can reasonably moderate a small board pretty well.
Normally you might ban someone and then they're back, but if you ban an account, they won't be back until they can get a new account.
The site users themselves can help with moderation. There could be a report button on each post, and if enough people click it, the system can automatically remove the post from view. Maybe only accounts that have gone through a week long probation period can file reports. That way nobody can try to get a bunch of accounts together and mass report everything. Just throwing ideas out here…