Assumption: a credibly neutral decentralized social networking protocol is desirable.
In order to achieve "credibly neutral decentralization" for the protocol data it must be 1) replicated across many servers / nodes 2) user-controlled, i.e. the user does not have to rely on any other party to add / remove messages to the protocol 3) sign ups are permissionless.
This means that anyone (or AI) can sign up for an account and post whatever they want at the protocol level. Any compromise on this negates the goal of "credibly neutral decentralization" for the protocol data.
However, just because the protocol itself is credibly neutral and decentralized does not mean clients / apps have to be. In fact, opinionated filtering is desirable from a user experience point of view.
So the practical end state: protocol is as neutral as possible, clients / apps get to choose what to show from the protocol, building a client is permissionless.
This means raw protocol metrics are always going to have noise. This applies to any permissionless protocol, including the web, email, Ethereum, etc.
Ethereum (arguably) has less spam than the web or email because it charges a small fee to use the network. Farcaster does this as well, but just upfront and then limits how much storage you can use of the network at any point.
Aside: related to #5, we publish the (opinionated) Warpcast spam labels for the benefit of other developers. But no one is required to use them. @neynar and @openrank have built their own scoring model. Multiple spam models is a good thing for the protocol.