We moderate a support chatroom for people with Asperger and autism. We experience that some users are disturbed by the conversations of other users, when those conversations touch on topics such as drugs or pornography. We don't want to outright ban those topics from being discussed, as some of our users may have drugs proscribed by their doctor, experience that medical marijuana is helpful, or struggle with a drug addiction. However, sometimes the tone is particularly disrespectful, or users are outright telling others that they should try drugs. When we point this out, users may complain that the rules we're enforcing are too subjective, or might ask "where in the rules does it say we cannot do X". Several users have broken out of the main channel and started their own channels, because they feel the main channel is too restrictive. The latter is not necessarily a problem, except that they and others remain unhappy in the main channel and challenge operator actions that they perceive as arbitrary or unjust.

I think trying to catch it all in a set of set-in-stone rules will not work. 
Rather, I believe we may need a code of conduct.

How would we start to write a code of conduct for an Asperger/autism support chatroom? How do we express in this code that we wish to remain friendly and open to visitors and new members, without alienating older users who think we're being too restrictive (and say so in no unkind terms)?