San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994

SECTION II : METHODS OF WARFARE

Blockade

93. A blockade shall be declared and notified to all belligerents and neutral States.

94. The declaration shall specify the commencement, duration, location, and extent of the blockade and the period within which vessels of neutral States may leave the blockaded coastline.

95. A blockade must be effective. The question whether a blockade is effective is a question of fact.

96. The force maintaining the blockade may be stationed at a distance determined by military requirements.

97. A blockade may be enforced and maintained by a combination of legitimate methods and means of warfare provided this combination does not result in acts inconsistent with the rules set out in this document.

98. Merchant vessels believed on reasonable grounds to be breaching a blockade may be captured. Merchant vessels which, after prior warning, clearly resist capture may be attacked.

and

SECTION VI : CAPTURE OF NEUTRAL MERCHANT VESSELS AND GOODS

146. Neutral merchant vessels are subject to capture outside neutral waters if they are engaged in any of the activities referred to in paragraph 67 or if it is determined as a result of visit and search or by other means, that they:

* (a) are carrying contraband;
* (b ) are on a voyage especially undertaken with a view to the transport of individual passengers who are embodied in the armed forces of the enemy;
* (c ) are operating directly under enemy control, orders, charter, employment or direction;
* (d) present irregular or fraudulent documents, lack necessary documents, or destroy, deface or conceal documents;
* (e) are violating regulations established by a belligerent within the immediate area of naval operations; or
* (f) are breaching or attempting to breach a blockade.

The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic.