Mandatory Breathalyser Test at the Gate, with a regulatory limit. If you are over that limit, its a no board, no fly policy. There will be cries of foul, abuse of " human rights " etc etc et al... But hey, until people learn, tough, treat them like kids. It's something that is only going to get more prevalent as time goes by.
Once on board, Single Shots only and limit the number per person.
Yes, purchase duty free etc, but have it delivered directly to the airplane in the hold, or alternatively, all orders paid for in your departing airport can be collected at your arrival airport. Electronic pay point systems can easily be connected worldwide. Just collect on the way out ...