The Israeli government's reduction of fuel and electricity exports to Gaza has been termed a modern instance of collective punishment that violates Israel's obligations under the laws of war and the Fourth Geneva Convention. But this claim is nonsense, and makes a mockery of international law.
It conflates failure to aid with active criminal harm. Acts of war are launched daily against Israel from Hamas-run Gaza. Bombs are lobbed against Israeli cities (especially Sderot), resulting in official government rejoicing when an Israeli civilian is killed or maimed. Hamas denies Israel's right to exist, and has masterminded countless acts of war against military and civilian targets in Israel. The Jewish state has the uncontested right to defend itself against such acts of war.
The bar on collective punishment forbids the imposition of criminal or military penalties (imprisonment, death, etc) on some people for crimes committed by other individuals. But ceasing trade with a country is not inflicting a criminal or military penalty against that country's citizens, not least because those citizens have no entitlement to objects of trade that they have not yet purchased.
The U.S. quite legally froze trade with Iran after that country committed an act of war against the U.S. following the 1979 revolution. The U.S. blockade of Cuba after they installed nuclear missiles directed at the U.S. was not a collective punishment of the Cuban people, it was a non-violent act of war in self-defense.
The claim is Newspeak. The charge of collective punishment appropriately describes the situation of the innocent Israeli women and children slaughtered while going about their daily lives in homes, schools, on buses and at shopping malls. They are in large number the victims of Hamas' measures of collective punishment against Jews - which violate their most basic of human rights - life itself.
Indeed, Israel has targeted the perpetrators of these atrocities individually, entirely in conformity with its international obligations. When Israel kills such targets, precisely the people who have individually committed acts of war against Israel, it highlights the difference between legal force and collective punishment.