Nostr

You can buy anything, but being my follow.

That's why friends-of-friends could be useful too.

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There are people buying followers tho

Yes, but not 'my' follows.

And if some of my follows starts following some paying spamming user, he will stop being my follow.

Easy.

I don’t see how this would be significant improvement over just blocking people manually

The difference is:

Opt-in vs opt-out.