https://github.com/disarticulate/y-webrtc/blob/master/src/y-... has a validMessage function passed into the room. This allows you to validate any update and reject them. It might be "costly", but it lets you inspect the next object. Since Yjs doesn't care about order or operations, it doesn't really matter how long validation takes.

Not sure what the error conditions loop like, but you could probably bootstrap message hashes in a metadata array in the object, along with encryption signatures to prevent unwanted updates to objects.