I am not a lawyer, but it seems all you can legally do when someone uploads CP to your server is delete it (fast). Hashing it (feds might consider that archival) or passing it around for purposes of a voting system(!) appears to be illegal.

This is so crazy and I have no idea how we can protect nostr from it if any reference is illegal and only centralized institutions are allowed to deal with it at all. Hashes sound like a reasonable thing to collect upon deleting stuff as it could be used to avoid displaying it to other users. Sadly hashes can indeed lead to discovery in content-addressable systems and of course, marking stuff as CP is a pretty sure way to keep people from opening it even if it's not actually CP.