A couple weeks ago someone was sending Hello Nostr from new empty accounts every second, so global feed was useless. Every message contained some POW even. It wasn't really malicious though - it didn't try to send to every single relay, didn't change the content, and didn't send 100x as much. So a couple relays were banned by clients, and "Hello Nostr :)" was blacklisted, and then the spammer turned it off.

Then there was a guy who was thrashing another one by answering to random threads from several accounts, and no one had any good idea of how to fight that. 

The only reason you don't see that every single day is because no one is doing it atm. It's just a question of time until nostr annoys someone and it comes back.