Leverage whatsapp and hang good old posters around the neighborhood?

https://github.com/TecharoHQ/anubis

https://selfh.st/apps/?alternative=IFTTT

Did you ask an AI to do the list for you? (no need to answer)

Intriguing.

What’s the mechanism for dealing with spammers?

In lemmy there’s a clear escalation path that will lead to either the spammer’s instance dealing with the issue or the instance itself being de-federated.

How would that work in a p2p system?

Each user having to individually block every spammer will work as well as it did for email back in the day.

Look at restic and/or borg first. Iff you have specific needs that they don’t address, then look at other things (including reviewing your requirements: chances are you are doing something wrong in that case).