You can’t ban at the protocol level, which is the great feature of nostr. Clients and relays can cater to customers however they want, which is the free market approach
Discussion
that's beauty core feature openness - relay admin has right block/filter/rate-limit whatever they think is harmful/illegal/DOSing-resources
if anyone think npub is BLOCKED by particular admin bc of political or technicall BIAS - just do ur own relay or move onto another relay so simple