One feedback item I received at Wikimania Hongkong 2013 and the Lyon Hackathon 2015 was that "No photos" stickers were handed out but basically ignored.
Not sure what's the best approach here when it comes to privacy vs. openness.
- announce in the opening session that such stickers exist and that photographers are supposed to respect them by explicitly and beforehand asking folks they would take a photo (or video) of
- make any external photographers / video folks (local TV etc.) who did not attend the opening session explicitly aware of those stickers and their meaning and the expectation to respect the privacy wish of those attendees
- or just don't hand out such placebo stickers if they will get ignored anyway