Private interlopers were subject to criminal penalties, and the companies fought wars in the 17th century to delineate and defend their monopoly territories.
The early 19th century Gibbons v. Ogden case weakened the steamboat monopoly that New York had granted, producing an exception for interstate commerce.
However the later Slaughter-House Cases established that a local law creating a legal monopoly did not violate the rights of other merchants in the United States.
The National Recovery Act to promote and legally enforce producer cartels was defeated in Schechter Poultry Corp. v. United States.
As do the Post Office departments in many countries, the United States Postal Service has a legal monopoly on delivery of non-overnight letters.