Moderating a forum where anyone can post is playing whack-a-mole, especially if registering the new account is simple.
One possible approach is something like Stack Exchange does: new users acquire their rights gradually. New accounts can only do little damage (post an answer that appears at the bottom of the list, and is made even less visible when someone downvotes it), and if they produce bad content, they will never acquire more rights.
Another possible approach would be some vouching system: moderator invites their friends, the friends can invite their friends, everyone needs to have a sponsor. (You can retract your invitation of someone, and unless someone else becomes their sponsor, they lose access. You can proactively become a co-sponsor of existing users. Users inactive for one year automatically retract all their invitations.) When a user is banned, their sponsor also suffers some penalty, such as losing the right to invite people for one year.
There are probably other solutions. The idea is that accounts that were easy to create should be even easier to remove.
For moderated deliberation to achieve consensus in decision making, here's a write-up for a system that combines ideas from StackOverflow, Reddit, and Wikipedia:
I don't think the issue is that it is too easy to change identities. Vouching or slow starts both lead to much more closed systems. You could say that is a solution for the posed problem. "A more closed system".
But to me, a more closed system is less valuable. Certainly it lacks the network effects that seem to be needed these days to make it financially.
The feature I'm talking about is direct vs indirect trust in WOT. If an indirect trustee is deemed untrustworthy, it may say something about the chain of direct trust links leading to him. If someone I introduce into the trustee network turns out malicious, I might get somehow penalized, as well as others I introduced. I realise WOT is designed with authentication in mind, but maybe it could serve more general trust systems. Not sure.
One possible approach is something like Stack Exchange does: new users acquire their rights gradually. New accounts can only do little damage (post an answer that appears at the bottom of the list, and is made even less visible when someone downvotes it), and if they produce bad content, they will never acquire more rights.
Another possible approach would be some vouching system: moderator invites their friends, the friends can invite their friends, everyone needs to have a sponsor. (You can retract your invitation of someone, and unless someone else becomes their sponsor, they lose access. You can proactively become a co-sponsor of existing users. Users inactive for one year automatically retract all their invitations.) When a user is banned, their sponsor also suffers some penalty, such as losing the right to invite people for one year.
There are probably other solutions. The idea is that accounts that were easy to create should be even easier to remove.