NIP-42 and POW both that prevent that  but allowing non-auth npub is also important for growth of nostr. 
no normies even know how to use nostr and asking everyone to use NIp42 client is make no normies use nostr.
so RATE LIMIT is still the key 

 normies won't have anything to come to if the network is broken 

 PoW, or fees, either way

we have lightning network integrations everywhere

PoW will only last as long as it remains more expensive or complicated, after that, only subscription fees will stop them

bitcoin survives entirely because it costs more to attack it than the benefits it provides 

 fully subscription only based will kill NOSTR from new comers evenb before it is born by that approach
there are several FREE image uploading n  txt gossip services to attract normies besides nostr.