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.