Yeah, this is the one thing that should maybe come with a bigger flag for new users. It's online forever. Some clients (e.g. Iris) have a 'delete' button, but as I understand it, that just adds a 'deleted' tag to the original post to tell clients not to display it.
Discussion
A note (post) cannot be updated in any way. A deletion is a new event (kind 5) that signal the note(s) deletion. The client should delete or hide the original note, but for experience this happens rarely. Some clients that manage the deletion just append a visual flag.
Full specs:
Thanks daniele.