After refining my thoughts, I think I see the problem:
1: The Banner AI must ban all transmissions of naughty Material X.
1a: Presumably, the Banner must also ban all transmissions of encrypted naughty Material X.
2: The people the Banner AI is trying to ban from sending naughty transmissions have an entire field of thought (knowledge of human values) the AI is not allowed to take into account: It is secret.
3: Presumably, the Banner AI has to allow some transmissions. It can't just shut down all communications.
Edit: 4: The Banner AI needs a perfect success rate. High numbers like 97.25% recognition are not sufficient. I was previously presuming this without stating it, hence my edit.
I think fulfilling all of these criteria with sufficiently clever people is impossible or nearly so. If it is possible, it strikes me as highly difficult, unless I'm making a fundamental error in my understanding of encryption theory.
To construct a friendly AI, you need to be able to make vague concepts crystal clear, cutting reality at the joints when those joints are obscure and fractal - and them implement a system that implements that cut.
There are lots of suggestions on how to do this, and a lot of work in the area. But having been over the same turf again and again, it's possible we've got a bit stuck in a rut. So to generate new suggestions, I'm proposing that we look at a vaguely analogous but distinctly different question: how would you ban porn?
Suppose you're put in change of some government and/or legal system, and you need to ban pornography, and see that the ban is implemented. Pornography is the problem, not eroticism. So a lonely lower-class guy wanking off to "Fuck Slaves of the Caribbean XIV" in a Pussycat Theatre is completely off. But a middle-class couple experiencing a delicious frisson when they see a nude version of "Pirates of Penzance" at the Met is perfectly fine - commendable, even.
The distinction between the two case is certainly not easy to spell out, and many are reduced to saying the equivalent of "I know it when I see it" when defining pornography. In terms of AI, this is equivalent with "value loading": refining the AI's values through interactions with human decision makers, who answer questions about edge cases and examples and serve as "learned judges" for the AI's concepts. But suppose that approach was not available to you - what methods would you implement to distinguish between pornography and eroticism, and ban one but not the other? Sufficiently clear that a scriptwriter would know exactly what they need to cut or add to a movie in order to move it from one category to the other? What if the nude "Pirates of of Penzance" was at a Pussycat Theatre and "Fuck Slaves of the Caribbean XIV" was at the Met?
To get maximal creativity, it's best to ignore the ultimate aim of the exercise (to find inspirations for methods that could be adapted to AI) and just focus on the problem itself. Is it even possible to get a reasonable solution to this question - a question much simpler than designing a FAI?