From wikipedia

A blockade is any effort to prevent supplies, troops, information or aid from reaching an opposing force. Blockades are the cornerstone to nearly all military campaigns and the tool of choice for economic warfare on an opposing nation. The International Criminal Court plans to include blockades against coasts and ports in its list of acts of war in 2009.

Blockades can take any number of forms from a simple garrison of troops along a main roadway to utilizing dozens or hundreds of surface combatant ships in securing a harbor, denying its use to the enemy, and even in cutting off or jamming broadcast signals from radio or television. As a military operation, blockades have been known to be the deciding factor in winning or losing a war.