Why does spam exist?

It only exists because the following three things are true:

1. People with malicious intent exist.

2. People can create new Nostr identities for free.

3. New identities can write to relays for free with the expectation that their notes will be seen and may not be removed for a time.

It is not possible to change 1, and 2 cannot be changed without a central authority that issues "approved" Nostr keys. Not a road we want to go down.

That means the only way to fight spam is by improving the ability of relays to block spam from being written to them in the first place and/or recognize and remove it quickly.

It is then the responsibility of users to only read from relays that do so. This means they need to have a place to find such relays. A relay marketplace, of sorts, that they can be referred to or that can be displayed within their Nostr client.

Clients aren't entirely off the hook here either. Defaults matter, and clients should have sensible default relays for new users that haven't built a relay list of their own yet. But they can only do so when there are solid relay options that allow write access to new users while also effectively recognizing and removing spam.

Clients can also provide tools for mitigating spam that do not rely on relays doing so, such as the way Coracle, Damus, and Nostur can filter posts based on WoT, regardless of what relays they use. Likewise, Amethyst has a toggle to hide posts that have been reported by those you follow. With these types of tools, though, your experience will vary widely from client to client, based on whether they implement similar tools and whether they toggle them on by default.

Meanwhile, picking good relays will affect your experience on all clients. Well... at least the ones that respect your relay choices...