WASHINGTON - Laurence Canter and Martha S. Siegel are certainly persistent.  They are the two Phoenix lawyers who blanketed the free bulletin boards of Internet with advertising, setting off a barrage of complaints that they had violated the Internet code of conduct.
   Despite that reaction, Canter and Siegel have announced that they are setting up a company called "Cybersell" to help other businesses advertise on the Internet.
   Its mission: "to use the entire Internet, which is accessed by more than 30- million users and growing each day, as a new and lucrative market for the sale of products and services."
   The lawyers criticized an alternative that would set aside areas of Internet for commercial marketing.
   "The information superhighway, of which Internet is one of the first real conduits, is a rapidly unfolding public resource.  Nobody owns it," Siegel said.  She said Cybersll would follow "reasonable" restrictions on time, manner and place that ads can be placed, but added "advertising has a role to play" on the Internet.