The CEO of a massive advertising company said it took him 4 hours to decide whether to blacklist Breitbart

It's one of many advertising-tech companies that track you so advertisers can target you more accurately. It's one of the biggest rivals to Facebook and Google — so much so, it's reportedly considering going public in the next few months.

So when AppNexus decides to kick a publisher off its platform, it matters.

Late last year, the company blacklisted the far-right news website Breitbart News, citing hate speech. The thinking is that advertisers shouldn't be supporting sites that sell hate speech, porn, or other inappropriate content.

Speaking with Business Insider, AppNexus CEO Brian O'Kelley said he had made the decision personally and that he had spent hours reading Breitbart's website to determine what was hate speech and what wasn't.

"I spent about four hours — it was actually a painful four hours — because I have political differences with Breitbart, but when I make these decisions, I work really hard not to let my own political bias play in," he said. "We all have biases, right? So some headlines were very hard to read, but they weren't hateful. Our definition of hate is more around the nature of speech, so whether it would incite violence against a particular minority. That bar is pretty high — it's not just nasty, or offensive, it's that [something] is inciting violence."

O'Kelley cited Breitbart articles about female genital mutilation and Muslim immigrants as examples.

He added: "I did a tonne of research to try and understand what that line would be, and because of the political implications in the US, you don't mess with Breitbart if you don't want reaction from the White House. It was a scary decision, and I thought we might lose a tonne of business over it. Fortunately, I don't think we did."

O'Kelley feared reprisals from the White House because US President Donald Trump's chief strategist, Steve Bannon, is a founding board member of Breitbart. The reprisals didn't happen, but AppNexus' communications vice president Josh Zeitz, also present in the interview with Business Insider, was the subject of at least one Breitbart article and, O'Kelley said, anti-Semitic abuse.

The article covered Zeitz's past work for Jon Corzine, who ran the derivatives broker MF Global before its controversial collapse in 2011.

"We do have a real obligation to use advertising to support some level of ethical media," O'Kelley said.

AppNexus has also put the conspiracy-theory site InfoWars on its blacklist.

Breitbart was never a direct AppNexus client, so the direct effect of the ban is negligible.

But O'Kelley said the ban encouraged other companies to follow suit. Breitbart had lost 1,000 advertisers as of February, according to analysis from the campaign group Sleeping Giants.

"We set an example that made it OK to ban them," he said. "The only major companies left that work with Breitbart are Amazon and Google."

This is backed up by analysis from Ghostery, a browser extension that flags ad trackers on any site, and shows Amazon Associates and Google AdSense trackers on Breitbart.

O'Kelley said it was difficult to try to police content without also restricting free speech, something Facebook and Google are also trying to deal with. When it comes to sites like Breitbart, he's seemingly unafraid to make his intent clear.

"I love the fact we can set an example," he said. "And even though we may not be big enough to completely destroy someone's revenue, you now see articles coming out about Breitbart rethinking its business model and considering being more mainstream."

Disclosure: AppNexus is an advertising partner of Business Insider's parent company, Axel Springer.