You would probably have to have some kind of flagging system built into the network, so that people could flag bits and pieces as childporn or whatever categories you provide. The client could be configured to refuse any data with user-selected flags.

Of course it would depend on the network not being taken over by a majority of childporn distributors and viewers, and the flagging system would need to be resistant to spoofing and manipulating.

Tribler is heading that way with anonymous metadata, iirc their roadmap.