A multi-tiered blocking approach at the server level is certainly better than what's being done now, but ultimately I think pay-to-write is the solution. Spam requires scale, and having to pay even a small amount to write to a relay destroys that scale and the incentive to spam.

I don't think expecting the client to do the filtering is realistic. Most clients are mobiles with limited bandwidth. Downloading predominantly spam events from a dozen relays and filtering through it all to get the few good notes is a tax on both bandwidth and the limited device cpu resources.

Filtering out spam at the server level makes more sense. The server has access to the entire db of events and can build metrics and algorithms that better deal with the spam. That's easier said than done, of course, but more doable than on a device with access only to the events coming in through the stream. But again, I think pay-to-write stops the problem and lets developers focus on features.