In my view publically announcing a firm policy to divert is the last thing you want to do. It would put me off flying with you, firstly I want no part of a diversion, and secondly the suggestion is that the airline needs to have such a policy. And sooner or later some idiot is going to test you to see if you really mean it.
Having a brethalyser at the gate could be effective. If you blow more than 1.0, your not flying today; more than 0.5 welcome on board, but we wont be serving you any booze, and we will carefully store any booze you intended to take on board, as well as the stuff your mates brought.
Additional training for FAs to help them deal with ugly situations would be a bonus. And you need big, butch, FAs that can intimidate any misbehaving cretins, something you rarely see these days.