yep, it is the lowest cost spam limiter

any spam limiting with a higher cost than calculating social graphs might as well just charge for relay access, you see?

even simpler filtering for paid relays can literally be, only accepting events signed by paid subscribers, or you can make a second level with them and their follows, or you can go further and compute those graphs and update them every time a follow event shows up that was let through the filter, and the administrator can decide on an arbitrary accept threshold to apply, according to their stomach for fluff - but it might also be preferable for the users if they like to get off the beaten path a bit more

these are all pretty cheap computationally, for the most part up to more than 4 billion users a 64 bit fingerprint of npubs is conflict resistant enough so it's really a small amount of memory being burned too