It’s your responsibility, paid relays, or you use your own relay, remove all other relays. Both approaches end in smaller reachability though…
Discussion
I'd imagine with that approach, a blacklist of scam npubs can be developed and sold/shared
Defies the purpose of a decentralized network imho and has the potential for abuse or false positives. But yes, you can try and gather enough people to use that service and those relays. But it’s only effective if you don’t use other relays. You can also create a private relay on an invitation basis.