What should you do to decrease teens eating fast food?

torture them if they will eat fast foods again/make a law against teens eating fast foods

xD

That isn't me on this answer. Try

1.Making it cheaper (budget)

2.Making it taste and smell better (addiction to good food habbits)

3. Or, with 95% chance of failing and making this 90x harder,

the rebellious do the exact opposite, tell them to eat fast food and ground them from good food. They might sneak out and eat regular food, thinking they're dissobeying you. They will try to not let you know, of course, but just hope and see if they get fatter or skinnier.