More spam solutions will be coming to nostr too. Here are a few:

paid relays - if you only read from paid relays, and they work at removing spam somehow.

filter scripts - if you can think of a way to characterize notes you don't want (e.g. says "AirDrop") scripts in clients can drop these notes. Gossip is close to having this.

PoW - IMHO not a good solution as spammers can PoW just like everybody else can.

social distance - If the person is not followed by someone you follow, don't show those notes

classifying people into more buckets - if clients allow users to mark a person as "recognized but not followed", then this can grow their social graph by a lot, and the social distance algorithm won't prune out quite so many people. I intend with gossip to have a thumbs-up/thumbs-down button on people you don't follow so you can "recognize" that they are a regular person not a spambot, without following them.