I guess the postal service is a good analogy, ‘we just deliver what folk post’. You don’t sign the letter - you’re anonymous. There are laws governing the letter contents, idk if they are the same as a fb message. But sending letters requires effort and FB trolls are a whimsical bunch, for a start you have to know what to put on the envelope. Perhaps it’s the financial cost of posting letters that puts people off, also probably more so it’s the effort and delay.
Anyway, this specific problem is trivially easy to solve - whitelist(allow list) corespondents: ‘you have a message from a new corespondent: ‘Anon Meanie’ would you like a preview of the message (it likely contains unfriendly content)? Or shall I bin it?’.