In general I think the answer could be pretty simple: dedicated marketplaces for products and services, where we go to search for the things we need and want. A humble newspaper contains great examples of good and bad advertising.

Newspapers have whole pages of bad ads, and random bad ads wedged between actual content. Ads have a perverse incentive to mimic the look of actual content, just like on the web. I'd never pick up a newspaper with a goal of "I want to find a tax service" and yet ads for such services are there, unwanted, wedged into other content.

But newspapers also have classified sections, a better kind of ad. They're in a predictable place, where you can go if you need a job.

Imagine if the actual content weren't perforated by a scattershot of ads. Ad revenue would go down, but readership would likely go up. Besides profit motives, it's also a case of the good of the many outweighing the good of the few.