ACROSS THE country, thousands of large and small firms—and government agencies—are installing special software that can block access to individual Web sites from employees’computers. And while many companies are only blocking the obvious suspects, such as sites featuring pornography, gambling or hate speech, some firms are taking matters a step further.

　　At financial powerhouse Merrill Lynch & Co., 30 to 40 percent of Internet surfing between 9 a.m. and 5 p.m. is not work related.