You can't stop someone from hosting it on their own relay.
Discussion
And this is a problem eventually
No. You can't stop someone from using email or the Internet, either. You can only refuse to host their traffic.
That is inherent to a true protocol.
So you say that this specific relay could become like an isolated island or something? What's the difference with lemmy and the fediverse then? I thought that a polluted relay interacts with other relays anyway. No?