There is nothing cruel about using a flash, unless your an idiot and you don't use it right, as anebel said.
I used one with my old TB and it worked wonders for him and it almost completely changed his attitude and afterward, we only used it ocasionaly as a reminder.
I hate to tell you but you're all basically wrong. The actual purpose of a flash nose band is to quiet the jingling of a loose ring snaffle.
Some trainers incorrectly over tighten them to shut the horse's mouth. But this is wrong. I use a cavesson with flash on my horse because I ride him in a loose ring snaffle. The flash keeps the bit quiet so that when I DO aid my horse he feels it really clearly and so all I need to do to ask for flexion is move 1 finger.
Again, just because there are a few bad apples that use tack incorrectly doesn't mean that the orchard is all rotten.
The nose band that WAS invented to shut a horse's face is the figure 8 nose band.
This is exactly what I use it for. I don't tighten it up either. It's just there to support the loose ring and prevent the loose ring from jingling up and down when we're riding.