You Choose who can post on your thread. My suggestion is the ability to control your own thread. Self clean and allow who can post on your own thread.
Each poster would in fact act like a mini mod. this will stop hijacking, spamming, trolling, harassment and attacks. These mini mods would only have control of a thread they start.
It would also stop double teaming.
This would not stop others from posting their own threads. So freedom of speech is preserved.
This is my suggestion. Not sure if coding would allow?