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Guys, in decentralization if you mean that no post will get removed and no topic or subject will get banned, then keep in mind that many people might also post some illegal and unwanted things. That will bring more problems.

Of course, for that reason peer-level-moderation is needed. Which means that yes, some content can be removed, and even should be removed. And who should do that?

1. Your own node. If you notice something ban-worthy, then you remove it from your node.

2. Your trusted peers. And by "peers", I don't think about your direct P2P connections, here and now, but rather the list of public keys of some unknown posters, which historically were good moderators, and which you decided to trust to some extent, to not be exposed into some very bad content. If someone would post naked photos, it is enough for one moderator to see that, it doesn't have to be shared with every node, if you trust that moderator. And you can still download that by default, if you decide to do so, you just don't have to watch that by default.

Which also means, that some unwanted messages will be probably relayed, but not displayed, if you choose to not display it. In this way, nothing will be "banned" (unless you, as a P2P peer, decide to drop a particular message, but then others may choose to broadcast it anyway).

So, when it comes to "banning" and "censorship", it is a different matter in decentralized forums. Because there are different kinds of ban:

1. Display-level ban: just a regular moderation. You decide to not display it. Or one of the "peer-moderators" you picked, decided to do so. But the message is still broadcasted over the network.

2. Relay-level ban: moderation enforced only by your node. It is similar to not relaying transactions below 10 satoshis per virtual byte. Your node can decide to do so, but others can pick some different rules.

There are other possible bans, but those two are the most common, if the network is used as intended. Which means for example, that you don't reuse public keys, and you don't know, who posted which message (unless you know master public key of that person, but even then, you can never be sure).