Microsoft says it has won its civil case against the Rustock botnet operators and is turning evidence it gathered over to the FBI for a possible criminal case.

In a blog post Thursday, Richard Domingues Boscovich, an attorney with the Microsoft Digital Crimes Unit, said Microsoft recently won its civil case to take down the notorious botnet, a collection of computers programmed by malware that sent out millions of spam emails:

Judge James L. Robart, of the U.S. District Court for the Western District of Washington ruled that the domain names and Internet protocol addresses used to host the botnet would be effectively removed from the defendants’ control. This case not only enabled the take down of a botnet known to be one of the single largest sources of spam on the Internet, but it is now helping to ensure that this botnet will never be used for cybercrime again.

Microsoft said that since March, the number of IP addressed infected by the botnet had been reduced by nearly 75 percent, from 1.6 million worldwide six months ago to about 422,000 the week of Sept. 11.

In March, Microsoft said it had brought down Rustock, a notorious international botnet the company says was responsible for sending billions of spam messages with pitches for Viagra and other prescription drugs. At its peak, the botnet was capable of sending out 30 billion spam emails a day.

Microsoft continues to offer up to a $250,000 reward for new information that leads to the arrest and conviction of operators of the Rustock botnet, a collection of computers programmed by malware to send out spam email.