Idea a) note staging/triaging relays 'noob relays': relays that accept all notes, filter spam from them and offer a concentrated remainder as a service (wot rateable, 3rd party auditable service)
Idea b) reverse auditing the timestamps of notes by an npub manually/visually (a visual representation of the time distribution may show obvious algoposting to human).  could be displayed alongside a visual representation of a word frequency analysis sourced from the npub's past N notes; perhaps deviation from typical vocabulary is a usable metric
Idea c) client sends a random bit of user data with the note.  i.e. how long did it take between final character typed and 'submit note' was clicked?  what deviation from straight line tangents were the mouse movements in the 3 seconds prior to posting?  Was the note's text copy/pasted into the note editing box, or was it typed?  Client sends just one of these bits of data to minimize making personal info public, but npubuser does not know which one will be sent, and so it is difficult to game, but helps to identify real users (and manual spammers).