Small relays don't need to accumulate spam.  I run a small chorus relay and only accept posts that tag me, and I moderate them for spam (which I haven't seen yet).

Nostr is no more or less censorable than the world-wide web.  Anybody can stand up a website.  Anybody can stand up a nostr relay.

If by "client relay owners" you are talking about clients that only talk to a relay managed by the client developer, then yes this is what many of us have been concerned about for a long time. Nostr doesn't require that model, and many of us have been pushing for a different model (the outbox model).