Let's say someone come up with a decentralized forum software where no authority can touch. That's pretty much what darkweb is. Every type of filth will leak from a place like that.
That's probably why there's not a single decentralized forum yet. And without centralized server, everyone will need to keep the entire database of all posts.
Theymos wrote about the possibility of a decentralized moderation system, kinda like the current Trust system. So if you trust me, you won't see posts I've deleted. But if someone creates a billion spam accounts, that's still going to be a problem. I expect to be stuck with a centralized forum for a very long time.