TimtimBT has a point. There is no really practical way to make junk food illegal. Trust me I am thinking about ways to tackle this problem too but I never said you could make those cheap bags of Doritos the more expensive option.
My solution: simply exempt fresh produce, meat, seafood, dairy, and grains from any sales tax so poor people (and those willing to save some extra bucks) will buy healthy food more often. Abolish this nasty "school meal" system in America run by large junk-food corporations and often get kids on horrendous diets right from the start. Replace them with better school meal systems such as "Natural Ovens" (google it if you have no clue what it is) and give them all the big contracts and responsibility of feeding America's youth.
Now in the supermarket department, simply making healthy food cheaper than junk food and making junk food expensive- via tax- (except chocolate because I am addicted to it XD) and taxing the junk food corporations while heavily subsidizing youth clubs, local farmers markets, organic farmers (certified), and banning growth hormones, artificial preservatives, flavors, and colors, GMO meat and dairy should help curb obesity.
Now I understand that not everything is solved by throwing money at it, but I think putting greater emphasis on nutritional education throughout elementary and high school (and education in general) should help with obesity as well.
Now that's better than simply "banning" junk food.
1- How would this be written? All sugars will be banned? Just the ones mixed with chocolate? This could cover an enormous variety of foods, and would be difficult at best to regulate well, as well as impossible to enforce.
2- Why should someone else's lack of self control be your problem? Why should you be punished because someone else can't get their act together? And sometimes those of us that have a bit of self control might like a snickers or something. What harm could that have, in moderation?