Haha, I saw the header and thought it was kind of weird that this was in psychology, but when I read through the question, I realized you actually had a very thoughtful question here.
Yes, it's well known that big food companies do things that are unethical: their processes are unsustainable and polluting, they bully small farmers, and many of their labour practices are disgusting (e.g. safety in slaughterhouses).
Not all large food companies are completely heartless, but most of them do at least a few things which most people would find objectionable.
So how can we use b-mod to change this?
Well, let me be honest. "Don't buy their product" isn't going to cut it. Any individual is too small. Even if you raise a stink with your friends and family and they grudgingly agree to boycott Del Monte, Wendy's, AND Chinese shrimp, the companies will not feel the difference in their profits.
I'm not saying you shouldn't eat ethically. I'm saying it's not enough to change their behaviour.
Punishing the companies who do wrong (from a psychology point of view it could also be negative reinforcement or extinction depending on how you think about it) requires some more organized advocacy.
One method is organized boycotts that mobilize thousands of consumers by publicizing the companies' wrongdoing. When guys write exposes about factory farming and go on the radio about it, that's great. You can help promote awareness about these issues.
The other piece to that, is raising a stink with the companies themselves. When you use b-mod on an 8-year-old boy, you tell him what you're doing and why, and what you want him to do. In the same way, you have to let the companies know (over and over) why you're trying to deliberately trying to hurt their bottom line, or they're unlikely to change.
Like the 8 year old, their first response will be to "greenwash"--to pretend to change while carrying out business as usual. Expose the greenwashing, and maintain the behavioural contingency (withholding the reinforcer--paying clients). Then maybe you'll see a change.
The other method is punishment. Mainly, law enforcement. Remember that in a democracy, you and I run the country. We just have to get the 10-300 million other people who run the country (depending on the particular country you live in) to see our point. If we can legislate and consistently enforce some good, fair, tough laws about food safety, marketing, pollution, etc. then the companies are likely to change their behaviour.
It's really just like trying to change the behaviour of an 8-year-old.