That's discriminatory. If a company told a fat person to leave their store, you better believe that that fat person would sue, and win a ton of money. Millions, if they didn't settle.
Interesting, my answer: Yes and No. Yes, because it does have a huge effect in accelerating the obesity rate. No, because I think the advertising biz is HUGE. Why just ban the advertising of food? Why not ban the advertising of cars- seeing as they cause car crashes. I don't know if that last example made sense, but still =D