NYT Article: Weight Loss By Manipulating Sense of Smell
There was an interesting NYT article on June 17, 2009 where some niche companies are claiming one can lose weight by saturating your sense of smell... either by enhancing or decreasing.
Read the article here.
Personally, I would think that decreasing your sense of smell would do more to help as this would decrease your sense of taste making food taste more like cardboard and/or sand.
There is a medical condition called "anosmia" where a person has completely lost their sense of smell. These individuals also are unable to really taste food as well. Anecdotally, most people with anosmia I've encountered are not overweight. Probably because nothing they eat tastes good... and so they eat what they must to not be hungry... and eat no more than they have to.