That's really it; they can't. There's no way to censor anything in the protocol just like there's no way to censor people's video cameras and flash drives. We can't look for a top down central or universal solution that automatically eliminates a specific class of content, just a cultural and social one where CP is hunted down and eradicated wherever it pops up because it's morally reprehensible.
Technology doesn't fix human problems. The CP argument has been leveled against every rising communications technology, but that's not the realm in which the problem can be solved. It's a social issue that needs a cultural solution.