You are correct that since nostr is censorship resistant, you can't really prevent someone from posting something, but you can prevent being exposed to it on your side. If it's a single nostr account (npub) sending you something you don't want, then you can block or mute them (the blocking is done in your app on your device). If they try attacking you at scale, then you can rely on web of trust (i.e. only allow content from people you actually follow, and 2nd degree) - this is now often the default.