Craigslist is cracking down on prostitution ads.

The Web site's operators reported an agreement with more than 40 attorneys general, as well as the National Center for Missing and Exploited Children, designed to block classified ads that promote human trafficking, child exploitation, and other illegal activities by members of its online communities. The Web site also filed 14 lawsuits against software sellers and service providers offering to circumvent the new system.

Craigslist CEO Jim Buckmaster met with Connecticut Attorney General Richard Blumenthal and representatives of NCMEC to develop the plan to stop people from posting the online prostitution ads, which most frequently appear under the site's "erotic services" category.

"Preventing site misuse and improving public safety are our highest priorities, and we are extremely appreciative of the encouragement we've received from the attorneys general and NCMEC," Buckmaster said in a statement last week. "The incidence of crime on Craigslist is actually exceedingly low, considering the tens of millions of legitimate ads posted each month by well-intentioned users. But no amount of criminal activity is acceptable, and as Craigslist has grown, we have become aware of instances where our free services were being misused to facilitate illegal activities."

Buckmaster said Craigslist is "unequivocally committed to stamping out misuse of the site and to improving safety for Craigslist users."

That includes businesses that sell software and services to evade the site's terms of use and defenses against misuse, including phone verification.

"By offering services designed to undermine Craigslist's ability to enforce its terms of use, these businesses facilitate the placement of ads for illegal services that would otherwise be blocked by Craigslist's protective measures," the Web site's operators said in a statement. "Craigslist has no tolerance for these activities."

The site has sent 14 "cease and desist" demands to companies and individuals offering the services and promised to cooperate with law enforcement investigating criminal activity on its pages.

Craigslist said it has refined protocols for blocking inappropriate postings and ads for illegal services. The site also has a flagging system that allows users to notify the site's operators of illegal and inappropriate content. The site deployed a rating system for tagging adult content so PCs with parental screening software can filter it out. Finally, Craigslist recently launched a telephone verification system for erotic services.

The system requires those posting ads in the section to provide a working phone number, answer a call, and enter a code onto the site before they can post such ads. It also creates a blacklist of phone numbers linked to inappropriate ads. Craigslist said phone verification reduces inappropriate ad volume by 80% and increased compliance with its terms of use.

Craigslist also announced plans to require credit card verification and a nominal fee to post "erotic services" ads for increased authentication and compliance.

"Requiring credit card verification and charging a fee to post in this category raises accountability to a point where we expect few illicit ads will remain," Buckmaster said. "More than ever, those who would misuse Craigslist to violate the law will find that Craigslist is a very inhospitable place."

The site said it will remove paid ads that violate guidelines and refuse to refund the fees. All net revenue from the ads will go to charity, and outside auditors will monitor the revenue and donations, according to Craigslist.