A clever idea at an event I recently attended: hand out strings of colored beads for people to wear if they do not want to be photographed. It seems like it solves the problem without requiring photographers to have a conversation with everyone who happens to walk through their shot.
@peterdrake Sounds like that should be the other way around: people wearing the beads do not mind being photographed. Takes away the problem when for some reason the beads aren’t visible in the photo and the photographer therefor assumes you don’t mind.
@peterdrake And makes it possible for anyone to see if the photographer made the correct choice.
>"...hand out strings of colored beads for people to wear if they do not want to be photographed...."
Like they do at Mardi Gras. 
@peterdrake
Great an easy/cheap idea- but still leaves discretion to the photographer to comply with the request.