I still think relay-based spam filtering is the way to go (some client-based filtering can also help, but shouldn't be the main thing), but there is not a clear way to do it:

nostr:nevent1qqsqqqra90st2ldev0fg9aurd3lheqgzajn8zw7820pqghfx6ca5m8cppemhxue69uhkummn9ekx7mp0qyghwumn8ghj7mn0wd68ytnhd9hx2tcpzdmhxue69uhk7enxvd5xz6tw9ec82c30nqs8v8

One obvious solution mentioned by nostr:npub1v0lxxxxutpvrelsksy8cdhgfux9l6a42hsj2qzquu2zk7vc9qnkszrqj49 that had never occurred to me was that clients should default to fetching replies from the OPs inbox relays and let them decide if they want their reply section to be filthy and full of spam or a nice place.