I think the simple solution is attacking a CS is an automatic DOW on their suzerain, but I agree with some others about flavor and agendas being off here. All the AI's currently just seem to operate the same way when it comes to CS which is to absorb them. It doesn't make for very fun game play and it gets tiresome after seeing it happen over and over again. I don't think many people would be in favor of a system where the AI is handicapped and the human player has free reign to take them out - just a more nuanced system where some AI civs like the Mongols aggressively go after them and others like Georgia try to protect them. Currently no AI civ actively tries to protect city states from what I have seen.