BACDS started trying to address this 10 or so years ago when we wanted to put dance images in our publicity and privacy concerns meant that we couldn't just use pictures.  It was very frustrating that while the Board was  arguing about this stuff pictures of our dances from random dancers were turning up on FB with no concern for privacy.  We eventually concluded that if we were going to take publicity photos we'd let people know beforehand, announce what dance in the evening we were going to do it in, and let people sit down or leave the room for that one dance.

That's okay when you're in charge of the photographer.

It turns out to be effectively impossible to really prevent photography without draconian measures like searches at the door and confiscating all the cellphones. 

The people who don't want to be photographed range from people who just have a preference to people for whom it's a matter of survival - those who have fled violent abusers or who are, say, in witness protection.  You cannot guarantee their safety and they need to understand that you can do at most a best-effort attempt.  

Given that, it's useful to designate one line as photos okay and have signage at the desk about not photographing outside of that line, but people will still miss or forget the signage, so organizers have to keep an eye on it and announcements have to be made, which may on any given night be for the benefit of people who aren't there.

I don't think we've tried giving people "no photos please" badges.  That might be effective in that people reviewing images before sharing them will see a reminder in the picture​, but it's not likely to be useful for people who livestream a minute of your dance.

-- Alan

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