Defeats the whole purpose of decentralization
Discussion
I do not see that blacklisting a spammer is making nostr centralized. Maybe a form of censorship but then paid relays are also a form of censorship. I see paid relays only as a form of relay sustainability (keep them up and paid for) Spammers will aslo pay sats if the value of their target market is high enough.
Blacklisting is useless, spammers can keep making new accounts. Paid relays will make it more expensive.
Spammers can also join paid relays. Paid relays add spam resistance. My thinking is more of paid relay with a referral system. Paid keeps the lights on and referral gives trust of not being a spammer.