It would be against the principles of the anti-censorship side of nostr but it would fit with the principles of the anti-being-censored-but-pro-censoring-others side of nostr

Web clients do block IP addresses from accessing them, any client that has a centralized element like a web client's website could do the same. Clients without centralized elements, solely relying on a transferable app and relays, can't really block users.

Any client that has access to your nsec can do some stuff to just fuck up your npub for you. A client could block everyone you follow and send constant spam to get you blocked too, until it loses access to your key. Browser extensions like nos2x can sign individual events to avoid this, or grant temporary signing permission to mitigate it, but nos2x itself could still hypothetically meet with some new malware code to do shit like that. You'd still ultimately be able to use the messed up npub though, or just switch to a new one