If you have the npub you can add it in the mute menu

You can't block anybody, that doesn't exist on Nostr because it's a decentralized protocol and content is served through multiple relays. Blocking (preventing someone from seeing your content) would require that ALL relays maintain a unified list of blocked users for every user. Not possible.

Muting is as trivial as adding the npub to your mute list. You client must have one somewhere in settings.

There is a menu where it says "Mute User". This is on nostrudel.ninja, but I'm sure in other clients there is something similar.

On Damus you can add npubs to your mute list in the settings

I think by unfollow him or amethyst have this