The communities moderate themselves with their own admins, just like on reddit. The difference is, there’s no global admins that can censor communities or enforce global rules. However, the plebbit app developer can basically act like a global admin by blacklisting connections to certain communities. I predict the most popular plebbit apps won’t include such blacklisting functions.

Plebbit is like BitTorrent, there’s no global BitTorrent admin. You use a BitTorrent client (like uTorrent) to download torrents, and the client could technically blacklist your torrent. You use a plebbit client (like Seedit) to download a subplebbit, and the client could technically blacklist your subplebbit.

It’s entirely possible that more centralized plebbit clients will be created, to be published on app stores for example, and they will implement whitelists of safe communities to participate in, blocking any other community.