Perhaps the members involved in the particular thread could be involved with the decision on whether or not a thread is being hijacked. That is not an easy idea to explore but I thought I would mention it.

If the thread starter remains active in the thread, it may be conceivable to permit them to request certain posts be removed. That may also be unfair. However, if one abuses that right by shutting out posts that others feel belonged, it would not take long for members who are offended by the thread starters judgement to understand that person has either bad judgement or is unfair and not waste their time participating in that persons threads.

That could also solve much of the disputes before they go too far. OTOH, it may cause resentment, but with strong personalities and wise judgement on the part of not only moderators, but other members as well, it could work.

I really do think that with enough brainstorming (I hate that word because it reminds me of my corporate years), we could make this happen.