Pricewert was thought to be home to several servers used to control computers infected with the Trojan program (also known as Pushdo). Criminals had been using these infected machines to pump out spam messages, and right before the takedown the ISP was responsible for about 30 percent of the spam tracked by Marshal8e6.

Last November, spam levels dropped close to 50 percent after notorious ISP McColo was taken off-line by its upstream providers, and it took months for spam levels to rebound to the same volume.

However, the results from the Pricewert takedown were not as dramatic.

According to data from Cisco Systems, spam levels at the end of last week but rebounded to normal levels on Sunday and Monday.

Security experts say that following the dramatic McColo incident, spammers may have put better backup systems in place to maintain control of their botnets of hacked computers. "Obviously, this was not a McColo. They were ready for the takedown," said Richard Cox, chief information officer with Spamhaus, an anti-spam group. "We've seen the backups pop up and have to get taken down and so on."