I think 'centralized' banning is a bit of a misnomer if relays are independently choosing to ban (spam or otherwise).  Even if every single relay happens to be independently making the same choice to ban based on the same criteria, the choice to ban or not is still decentralized.

*also, with regard to relays that are neither owned nor operated by the public, the term 'public relays' may be a bit misleading in the context of censorship.