Re: heard similar please before
I'm a very old git - probably old enough to be the dad of most commentards here. I remember US band The Grateful Dead who had a novel approach to this sort of problem.
The issue then wasn't fans with smartphones - given that they hadn't been invented - but with fans who were recording the gig on audio tape. The Dead's solution? They set up a roped-off area which had been carefully chosen to give "good sound" and invited fans to set up their recorders there. This wasn't a trick, the recorders were allowed to run and the fans came back and collected them after the gig.
When asked why they positively encouraged taping they argued that since fans would find a way to tape the gig anyway they would prefer that any recordings that were being circulated were of good quality and didn't dtract from their performance.