I don't think there's much you can do beyond spam filtering the filter relay because the only alternative would be to stop bridging NIP-28 which is probably one of the intended goals of the filter in the first place. I think the best solution would be for Amethyst to keep NIP-28 out of global IMHO or at user level just leave the defaulted Nostr and Amethyst group chats so spammers don't have those huge honeypot rooms to broadcast to.
Discussion
Filtering our aggregator is definitely on our to-do list. Strfry has an amazing update in pipeline that will let us run our own models on event data we are streaming from big relays as it comes in. There are some patchy ways to deal with it now but we wanted to start with a big net and refine.
Soon…
Thank you so much for this info btw, very useful.