nostr (the protocol) does not describe a functionality to prevent a user on the network from communicating and sending events to the network. in fact, it doesn't even dictates that can only be a single network. multiple can operate without ever overlapping.
however, clients, relays, and other users can FREELY chose NOT to communicate or propagate content. that is not censorship, that is rejection. being rejected can happen. there is no guarantee that every network will accept every kind of communication