I've seen the code of conduct thing work even before smartphones were ubiquitous -- even the open mic night at Eddie's Attic in Georgia was an event where the manager made it very clear that the venue was a Listening Room, not a place for talking over the performance, and if you wanted to use your phone in any noticeable way, you needed to step outside. And I saw him dress down and kick out a customer that decided to ignore the rule.

It's harder with larger crowds, but I think a policy with clear expectations that can result in you losing your privilege of attending is a better choice than asking people to check valuable (and useful) property at the door.