Freeganism is a total boycott of an economic system where the profit motive has eclipsed ethical considerations and where massively complex systems of productions ensure that all the products we buy will have detrimental impacts most of which we may never even consider. Thus, instead of avoiding the purchase of products from one bad company only to support another, we avoid buying anything to the greatest degree we are able.
And though this choice is not without some significant costs in creature comforts (garbage picking, etc.), it’s also a pretty good example of ideological resistance, or the capacity for it in our consumer economy vs. say having to stand in front of tank in order to vote.
Ideology has a lot of definition. In it’s most basic sense it means a “body of ideas” or a “worldview,” but even when people use it causally, they generally mean more than just any idea. They usually mean an idea that expresses some form of social or economic power. In this way, it still carries the Marxist implication of “dominant ideology” i.e. a set of ideas that is designed to make the interests of ruling class appear as the interests of everyone. Or to paraphrase Althusser, “a fantastic relation to the real material conditions of existence.” It’s a message or image or set of ideas that makes us believe something about role in the world contrary to the material facts of our lives. (Wikipedia isn't bad on the subject. Here is a summary of some of the key terms in a popular culture class at Georgetown)
By any of these definitions, all advertising is ideological: the whole point of advertising is to reproduce the desire that drives the consumer economy. It would almost be impossible for advertising to not be ideological. So, when academics or marketers talk about the ideological impact of advertising, I have to agree, because what else could it possibly be? If it failed to be ideological in some minimal way, it would have to be some really poor advertising.