not a trick question, i'm just dead convinced that the economic reality of "per message cost" is doomed in broadcast networks. it would only work e2e.
Discussion
so no better idea
i truly wish i had. in general to combat spam in a broadcast / flood network you need to make the identity costly and block them before the spam fully propagates. which can work with persistent pseudo identities, but i think you have more ephemeral identities here by design.
i wrote about my thoughts on the subject in more detail here: