“This is a huge issue that Google needs to tackle now,” said Craig Fagan, program director at the Global Disinformation Index. “It is creating a financial incentive for these websites to continue promoting the conspiracy theories. You go to these sites and there are ads galore, pop ups everywhere. The ads are there to get clicks, monetizing each reader.”

In one case, Google accepted ad revenue from a company promoting a conspiracy theorist it tried to remove from its own platforms.

In early May, YouTube removed the account of David Icke, a British provocateur who often ranted about "Rothschild Zionists" controlling global institutions and has questioned the efficacy of vaccines. In a recent interview about Covid-19, he said that 5G makes people sick and sends out signals that can control their emotions. Icke had posted on YouTube for more than 14 years.

Guillaume Chaslot, a former Google engineer and founder of the research group AlgoTransparency, estimated that Icke’s YouTube channel gained 200,000 subscribers during March and April, when he largely touted unproven theories about the virus. Chaslot's research tracks how often YouTube's recommendation system sends viewers to particular videos and channels. In a 10-year span, YouTube promoted Icke's videos about a billion times.

YouTube removed Icke’s account for violating its rules about coronavirus disinformation. Since then, Icke has appeared on other YouTube channels and in YouTube ads for Gaia Inc., a streaming network that promotes yoga and alternative healing. "We have to break out of this perceptual prison," Icke said in a voice-over during an ad that ran weeks after his ban. Gaia's network runs several shows featuring Icke. On a recent earnings call, Gaia executives said YouTube had become a "pretty significant" way to get new subscribers.

Gaia didn’t respond to requests for comment.

Imran Ahmed, chief executive officer of the Center for Countering Digital Hate, a U.K. nonprofit, argues that social media platforms should remove Icke entirely. “In a pandemic, lies cost lives," said Ahmed. "Misinformed people put us all at risk through their reckless actions.” His group estimated that Icke earned about $177,000 a year from YouTube ads before the ban.

Jaymie Icke, a spokesman for Icke's video service Ickonic, said the earnings estimate was inaccurate because YouTube has restricted ads on controversial videos for several years. "Revenue is nothing and has been for a while," said Icke, who is David Icke’s son. "They removed all ads from the channel two months prior to the full deletion anyway. So that figure has simply been made up."

Icke and others blocked from the site are allowed to appear on other accounts and in ads as long as those videos don't break rules, according to Muldoon, the Google spokesperson.

While web giants like Google have tried to handle conspiracy theories on their user-generated services, they have also tried to reform their ad systems to handle the growing problem. In October 2018, Google and Facebook Inc. signed a European Union code of conduct on disinformation that contained a commitment to “improve the scrutiny of advertisement placements to reduce revenues of the purveyors of disinformation.”