A growing number of advertisers in Canada are feeling the heat for placing ads on The Rebel, a website founded by Ezra Levant as a successor of sorts to the now-defunct Sun News Network.

Mr. Levant has said he took some inspiration from Breitbart.com, the right-wing, often caustic site whose former executive, Stephen Bannon, is now an adviser to U.S. President Donald Trump. Now, Mr. Levant's site is the subject of the same kind of social-media campaign that pressured companies to pull ads from Breitbart.

Sleeping Giants, an anonymous group, created a Twitter account in November to publish screen shots of ads on Breitbart and to call out those advertisers for appearing there. In February, an affiliated account was created for Canada, and the group began focusing on The Rebel's advertisers. The Canadian account has used social media to pressure brands such as 7-Eleven, Dynamite clothing stores, PetSmart, the Royal Canadian Mint, the NCAA, BMW Canada – for an ad placed by one of its dealers – and others.

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Earlier this month, B.C. ski resort Whistler Blackcomb, owned by Utah-based Vail Resorts Inc., confirmed to the National Post that it had pulled its ads from the site. In response, The Rebel launched a campaign encouraging its readers to boycott the resort. "All we have to go on here is the public virtue-signalling by a few junior Maoists bad-mouthing their own company's customers as being politically unhygienic," Mr. Levant said in an e-mail.

The campaign, and marketers' responses, shed light on a larger issue in the digital advertising economy. Advertisers used to pick and choose the media environments where they appeared in the hope that the audience they wanted to reach would be there. But now, buying technology lets brands follow potential customers around the Web, targeting them more directly in a variety of contexts. So different ads will appear on the same site depending on who is looking at it. Partly because that buying is done more and more with automated systems – known as "programmatic" advertising – marketers are not always aware of where their ads end up. That was the case earlier this year when advertisers found that some ads on YouTube appeared alongside inappropriate or even extremist content.

Sears Canada, for example, was not aware that ads for its subsidiary, Corbeil Appliances, were showing up on The Rebel before the social-media campaign. Like many marketers, Sears and Corbeil give guidelines to their media-buying agencies – including blacklists of sites where they do not want to appear – and the company says its ads on The Rebel were an error.

"Definitely there are sites we avoid, such as the one in question. As stated earlier, we want to avoid sites that our customers might find to contain undesirable content or not fit with Sears values," said Vincent Power, Sears Canada vice-president of corporate affairs and communications.

A spokesperson for PetSmart said it had removed the site from its media buy.

Hudson's Bay confirmed it had "adjusted" its ad buying to prevent ads from appearing on the site, but did not explain why it came to that decision.

General Motors Canada also removed the site from its ad buying.

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"As we go to market with ads in the digital space, where they get placed has to abide by our brand standards," said George Saratlic, lead manager of product, brand and technology communications for GM Canada.

The same was true of the Royal Canadian Mint.

"Once we were informed of this particular ad, we consulted our agency, which informed us that this placement was inconsistent with the established guidelines used on our behalf and took appropriate action," director of affairs Christine Aquino said.

Advertisers have to be cautious about appearing in what are known as "brand safe" environments. The Rebel is a tricky fit. While much of its content is news and analysis with a conservative point of view – and as Fox News has proven in the United States, partisan content is not necessarily seen as unsafe for brands – other elements of the site are more questionable.

A recent video by contributor and former Sun News reporter Faith Goldy posed the question of whether demographic shifts in Canada's population due to immigration amount to "white genocide." Another story asked if "political correctness" was to blame for the suicide bombing in Manchester last week. And one of The Rebel's marquee contributors is Gavin McInnes, a co-founder of Vice Media who is no longer affiliated with that company. In a recent video, Mr. McInnes ridiculed a segment on Ellen in which a woman in a wheelchair danced with a partner, saying: "Who doesn't want to know a handicapped person? That's cooler than a black friend. I want to at least have a friend with, like, a lobster claw. You need that in your repertoire. Friends are baseball cards. You need some freaks in the mix."

Nova Scotia Liquor Corp. said the content of the site did not align with its values as a Crown corporation and pulled its ads – which had been placed via programmatic advertising – on Wednesday.

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"We do our best to avoid advertising on any sites that could be viewed as extreme or offensive to our customers," said Jennifer Gray, NSLC senior manager of communications. "Our values act as a lens to guide our business decisions, and advertising on this site just did not align."

Ottawa Tourism, which has also pulled its ads, received feedback from followers on social media.

"Tourism is supposed to make you feel good. … So we wanted to be in a positive space," spokesperson Jantine Van Kregten said.

In calling for the boycott of Whistler, Mr. Levant has said the resort is rejecting The Rebel's readers and viewers.

"Whistler-Blackcomb made it clear that the Rebel is simply 'not their kind,'" he said in an e-mail. He added that he has received a "large number" of e-mails from customers expressing their anger at the resort and, in some cases, cancelling vacation plans.

Because of the mechanics of digital advertising, however, brands are continuing to deliver ads to people who read The Rebel – just not on The Rebel website.

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"We're interested in people coming to Ottawa, whether they read The Rebel or not," Ms. Van Kregten said. "There are methods to get that message across in a more appropriate space."