Microsoft and law enforcement agencies said Thursday that they disrupted a botnet that defrauded online advertisers of US$2.7 million a month but that the malicious network hasn’t been completely eliminated.

The “ZeroAccess” botnet infected computers with malicious software that interfered with search results in a browser, directing people to websites where cybercriminals profited from bogus clicks on ads, according to a news release.

Microsoft, working with A10 Networks, the Federal Bureau of Investigation and Europol, the European Union’s law enforcement agency, said they did not expect to completely stop the botnet due to its complexity.

As it has done in other botnet interventions, Microsoft filed a civil suit in the U.S. District Court for the Western District of Texas against eight unnamed “John Doe” defendants.

The lawsuit, filed on Nov. 25, alleges that the defendants also used the infected computers to commit identify theft and DDoS (distributed denial-of-service) attacks. A notice announcing the lawsuit is written in both English and Russian, indicating the suspected language of some of the accused.

Online advertiser spend hit $20.1 billion in the U.S. in the first half of this year, according to the lawsuit. The industry’s “size and rapid growth combined with its highly technical and organizational complexity has made online advertising a rich environment for cybercriminals,” the suit said.

In click-fraud scams, advertisers end up paying for bogus clicks generated by software. The traffic from infected computers is sold by cybercriminals to other people running websites, who benefit by collecting fraudulent advertising revenue.

The U.S. federal court allowed Microsoft and investigators to block communication between the botnet and U.S.-based computers and take control of 49 domain names used by the botnet.

Europol, working with Latvia, Luxembourg, Switzerland, the Netherlands and Germany, executed search warrants and seizure orders on various computers related to 18 IP addresses connected with ZeroAccess.

As many as 1.9 million computers were infected with the botnet code in October, Microsoft said, citing research from the University of California at San Diego. About 800,000 computers infected with ZeroAccess are active on the Internet on a given day.

ZeroAccess disables security software on a computer, making it difficult to remove, Microsoft said. Microsoft has published general instructions for how people can keep their computer free of malware.