I suggest a forum that uses email to create posts and reply to them. When you click on a "create post" or "reply" link you are directed (via a capatcha of course) to a unique use-once email address. You email your post or reply to that email and it appears on the forum.
Your email address would not be displayed to avoid spam. This could lead to confusing fora, so this is not the perfect solution for all fora.
The fora can be moderated by blocking email addresses of abusers.