I just don't see how "junk food" can be defined in a fair way.
I say we allow me to define what's junk food and what's not. And no- it might not be fair, but it will be effective.
Pizza with chicken peperoni : Not Junk Food.
Pizza with pork or beef peperoni: Junk Food.
Frozen Yogurt with <3.5% fat : Not Junk Food.
Frozen Yogurt with > 3.51% fat: Junk Food.
Hamburger, BBQ'd with whole wheat bun: Not Junk food.
Hamburger, Fried with white bun: Junk food.