A federal judge on Sept. 10 threw out a Pennsylvania law requiring internet service providers (ISPs) to block web sites containing child pornography, saying the tools to do so also cause “massive suppression” of constitutionally protected speech.

Enacted in 2002, the law gave Pennsylvania’s attorney general the power to order companies like America Online to block customers from viewing web sites that the state had identified as containing illegal content.

No one challenged the state’s right to stop the distribution of child porn, which is already illegal under federal law, but lawyers for the Center for Democracy and Technology and the American Civil Liberties Union argued that the technology used to block web sites was clumsy.

Much as the phone company can’t control what people fax over phone lines, ISPs can’t control content on the web, and efforts to use sophisticated filters to stop people from seeing illicit sites have proven problematic.

Over two years, the groups said, ISPs trying to obey blocking orders were forced to cut access to at least 1.5 million legal web sites that had nothing to do with child pornography or even legal pornography, but shared internet addresses with the offending sites. When a service provider blocked the address for a child porn site, it wiped out the entire cluster.

U.S. District Judge Jan E. DuBois agreed the law could not be enforced without also blocking protected speech.

“There is little evidence that the act has reduced the production of child pornography or the child sexual abuse associated with its creation,” DuBois wrote. “On the other hand, there is an abundance of evidence that implementation of the act has resulted in massive suppression of speech protected by the First Amendment.”

Lawyers for the state had argued the technology exists for ISPs to block selectively and blamed internet companies for not wanting to upgrade their systems. The state said ISPs were making business decisions by choosing to go the cheaper, easier route of blocking thousands of sites sharing the same internet addresses.

Equipment is indeed available to shut down individual sites, but experts say such costly technology would force smaller ISPs out of business and larger ones to spend tens of millions of dollars on a weapon effective only until the peddlers of online child porn change tactics.