In the Nostr protocol, once a "note" (message) is published, it cannot be deleted in the traditional sense because Nostr operates on a decentralized network. Here's a simplified explanation :

Permanent Posting : When you publish a note, it is sent to relays (servers) that store it. You cannot directly remove it from these relays.

Deletion Markers : Nostr supports something called "deletion events," where you can publish a message indicating that you want a specific note to be deleted. However, this doesn't guarantee the actual removal because it's up to the individual relays to decide whether to honor that deletion request.

Relay Behavior : Some relays may respect deletion events and remove the note, but others might not, meaning the note could still exist on some servers.

So, while there is a mechanism for requesting deletion, notes on Nostr are generally considered permanent once they are published.