Iris already basically solved the spam problem a while ago, or at least mitigated it to the extent that it’s not all that bothersome.

If a profile isn’t already followed by X number of people (configurable) the posts won’t be public until you follow them.

You can’t stop spam on Nostr completely because accounts aren’t tied to instances, admittedly it’s already pretty extreme and the network is still tiny. But it’s not some unworkable thing either. Clients just need to implement filters like Iris, and relay operators need to prune sometimes so their database doesn’t get huge.