Here's an alternative. If the cameras will definitely be using flash, you could use an Image Fulgurator to add a message like "THIS PHOTO WAS TAKEN ILLEGALLY" to their photos.

A large majority of cameras, phones included have infrared filters built in, you'd upset night vision cams and that's it.

If you were really determined you could probably come up with some kind of light based dazzler, the only thing is it would also dazzle eyes.

What with proper cameras, iPods, phones etc, there are eight digital cameras in the Kite household.

None of them use infrared in their focusing, and I have used them in a hall warmed by three large IR heaters, and none of them were affected.

maybe a gallon of petrol and a match