Google has removed almost 600 “disruptive” Android apps from the Play Store and banned their developers in its latest effort to rein in mobile ad fraud, the company announced on Thursday. Google’s policy does not allow ads that display when an app is not in use or that trick users to click on ads by mistake.

The company says it defines disruptive ads as “ads that are displayed to users in unexpected ways, including impairing or interfering with the usability of device functions,” like a full-screen ad that pops up during a phone call or when using navigation apps, according to Per Bjorke, Google’s senior product manager for ad traffic quality who wrote the company blog post.

Bjorke also notes that the company had developed a “machine-learning based approach” to help it better detect out-of-context ads in apps, which led to today’s sweeping bans. “Malicious developers continue to become more savvy in deploying and masking disruptive ads, but we’ve developed new technologies of our own to protect against this behavior,” Bjorke writes.

Most of the apps found in violation were made by developers based in China, India, and Singapore, and a majority were aimed at English-speaking users, BuzzFeed News reports. Repeat offender Cheetah Mobile, a publicly traded company based in China, had more than 40 of its apps yanked from the Play Store in this latest wave of bans, according to BuzzFeed News.

Bjorke told BuzzFeed News that it appeared the offending developers had used similar techniques to evade detection, but he did not know if it was a coordinated effort. Google will offer refunds to brands whose ads may have been affected by the disruptive pop-ups, he added.

It’s not the first time Google has taken this kind of action against developers, although today’s action appeared to be its biggest sweep of ad fraud offenders to date. In July, Google banned Chinese developer CooTek for using an adware plug-in that sent users aggressive ads even when an app was not in use.