I'm wondering how decentralized systems based around ActivityPub plan to combat problems like spam and astroturfing? These seem to be easier to address in the

I'm wondering how decentralized systems based around ActivityPub plan to combat problems like spam and astroturfing? These seem to be easier to address in the walled garden platforms due to the administrators having a global view of the social networks. Decentralization on the surface seems to allow bad actors to more easily mask their activities from effective moderation.