I haven’t seen actual figures — this would probably be really difficult to prove — but I’d be willing to bet that banning smoking actually did a lot more than any ads to get at least some people to quit. You can ignore ads pretty easily, but it’s a little tougher to ignore a bartender telling you to take that cigarette outside. I have a friend who smokes socially, and the really disgusting ads that have been running lately in NYC have no effect on him. (Basically, they show photos of smokers’ lungs and squeeze the goop out of a dead smoker’s clogged arteries — they’re incredibly unsettling.)
I’m generally a "hands-off government" type of person, but on the smoking issue, I really, really love smoking bans, mostly for selfish reasons — I absolutely hated coming home reeking of cigarette smoke and sniffling after going to a bar with friends when I hadn’t smoked a single cigarette. It’s just so pleasant being able to breathe clean air.
Smoking is a little different from trans-fats, I think, because it produces more immediate negative externalities on other people, from the unpleasant (odors) to the unhealthful (diseases caused by second-hand smoke). I think that gives government a little more authority to regulate it.
Trans-fats don’t affect other people as directly; it’s more of a personal issue, so I’d be wary of banning them outright. That said, the more we move towards socializing medicine (I’m not saying that’s a good thing) in this country, the less this becomes just a personal issue and the more we’re going to have to legislate stuff like this, I think. Diabetes treatment/surgery/medication can get very costly. And, as some other posters pointed out, obesity isn’t caused by trans fats or HFCS alone. Trans fats are just one tiny part of the puzzle and banning them will likely not do much to improve obesity rates.
Anyway, that’s my two cents.