Demand it from our news sources and anything that presents itself as legitimate news. I
Put in a bit of work - make a list of companies who advertise on (for example) Breitbart, then divide that list into companies whose products you buy (or might buy), and those whose products don't interest you.
Write a letter (a letter, not an email, not a phone call) to the PR departments of those in the first category, and tell them that you will not buy their products while they advertise on Breitbart, and their competitors who don't advertise on Breitbart will get your $$$ instead.
A guideline for marketing used to state that for every 1 person motivated enough to actually do this, there's roughly 8 others who feel the same way who can't be bothered to complain, they just take their business elsewhere. If enough people write such letters, companies will respond, especially if it's backed up by falling sales - and maybe they'll do a Kellogg's on Breitbart. If there's one thing media companies respond to, it's loss of advertising.
You can write similar letters to the second category if you want to, but to preserve your own integrity, you shouldn't be lying about buying products you never had an intention of buying. Don't bring yourself down to their level.