Blog post by Christine Lemmer-Webber, co-editor of ActivityPub: https://dustycloud.org/blog/how-decentralized-is-bluesky/
The likely answer to this is that there will always have to be a large corporation at the heart of Bluesky/ATProto, and the network will have to rely on that corporation to do the work of abuse mitigation, particularly in terms of illegal content and spam. This may be a good enough solution for Bluesky’s purposes, but on the economics alone it’s going to be a centralized system that relies on trusting centralized authorities.
Bluesky allows you to block quote-tweets of your posts. That's already a good mitigating measure for this kind of harassment.
Another thing that Bluesky does well is blocklists. We should probably consider something similar here on Lemmy/Mbin, that would help to crowdsource moderation and reduce the workload on mods/admins.