Yea, so, on the actual topic.
In high-school, my auditorium was so sensitive to cellphones that if someone backstage was on the phone we could hear their conversation on the audio! Basically, we dealt with the cellphones as having a nice speech before the show that was somewhat comedic to get people to turn off their phones. Also, during the show, we had a walkie for the usher with a head-peice.
The ushers would stay at the top of the room and look out for screens in the audience. People who ran spot or techs that had no other job, would sit in the balcony or in the light booth (depending on if the balcony had people in it) and look out for screens too. At any notice of sound or screen the usher rushed out and asked the person to put away their phone. Also, at intermission we welcomed them back in again reiterating that cell phones were prohibited.
That's the main method we used, but if we missed some there's not much you can do. That person will feel embarrassed and hopefully others and they will learn.
Oh I almost forgot! One great method is to over-exaggerate how the cellphone will effect the sound system. It really did mess with ours, but we told them mics would fail and all sorts of things to scare them.