A combination approach seems to be working with smoking, high taxes, education and social pressure and legislation banning in public areas.
Take that to junk food you'll get increased taxation on high calorie/fat foods(sweets, crisps etc) restriction on granting planning permission for fast food outlets and education possibly scope for promoting excercise more.
However you can't lump everybody in the same boat. There are strong genetic obesity effects - OK they don't explain the general trend of massively increased obsesity but there are a significant number of people who are geneticly prone to obesity and no amount of dieting and excercise will make them Kate Moss.
You also shouldn't assume that somebody who is overweight is unfit.
http://www.forbes.com/health/2005/04/06/cx_lrl h_0406fitfat.html
It's more about how active you are than what you weigh