content: To be honest if the performers really wanted that enforced all they have to do is when they see a bunch of phones/tablets out stop performing and repeat that it is not allowed. Do this over and over and it will wreck the experience for the crowd and eventually people will start policing each other as they don't want the concert ruined, but be sure that you still perform the whole show you had planned. Yes there would be some backlash at first but just be honest with the public and point out the message that everyone sees and hears before every performance.