> The issue with that is that when there is no "leader", there is also no way to guarantee kicking someone out.

Why is that an issue? It's a fundamental fact about the world that your software will never address. No matter what options you purport to provide, you can't stop people from telling other people what messages they received.

In a decentralized system, messages are sent to a list of recipients. If you don't want someone to receive your message, you can take them off the list of recipients that you send to. But if you send a message to party B, and they recommunicate it to party C, there's nothing you can do about that. The only solutions are (1) to stop communicating with people you don't trust; or (2) to have the guy you want to kick out of the chat group kicked out of the world.