A trust based system could work, Nostr Band is basically trialling exactly that. Could also be a private relay where it's easy to get in but if you spam with unrelated content you're booted out.
For this to work though, a special event type dedicated to nsfw content would be necessary, or at the very least some type of standard adopted across all clients for posting certain notes to specific relays, because you can't expect even tech savvy users to have to disable all other relays manually, it's just poor UX.
Simple to implement on a technical level but getting everyone to agree on a standard to implement across the main clients is the difficult bit.