I hate to break this to you, but the only answer Nostr seems to have for combating spam is paid relays. Do a search to see how many essays people have written on the virtues of having to pay for relay access. Most relays either expect payment or are walled off entirely.

Not to say there aren't any other filters being developed client side for example, but the only reason Nostr isn't devolving into a spam fest is because nobody's really trying yet.