Nostr: What happens when I mute or block someone?

Short answer

Muting usually changes what your client shows you; it does not revoke another person's public key or prevent all relays from storing their events. NIP-51 defines mute-list data, but user-interface behavior varies by client. A relay or group may also apply its own moderation rules. If you need to prevent someone from reading a public post, muting is not enough.

Sources

NIP-51: Lists, NIP-29: Relay-based groups.