Eleven Southern Californians indicted last week on mail fraud charges relating to the operation of several fraudulent office supply companies pleaded not guilty in Los Angeles federal court. The companies allegedly collected $36 million between 1981 and 1985 by inducing small businesses to purchase over-priced photocopying supplies. Sheldon Block, president and owner of Park Distributing Inc., remained free on $1-million bond. U.S. Magistrate Ralph J. Geffen set bonds ranging from $15,000 to $300,000 for the others indicted. Two defendants, John Franklin Cox, 29, of Hollywood, and Vincent James Baca, 46, of Los Angeles, are fugitives, Assistant U.S. Atty. David Katz said. U.S. District Judge James M. Ideman set an April 1 trial date.