The first example that I know is about chemical weapons after the first world war.
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Even before the WW1, The Hague Conventions (1899 and 1907) worked, that, for example, forbade to use balloons for shooting or bombing or banned some rifle/cannon calibers.
Crossbows were forbidden several time by Catholic Church or some kings. It was in Middle Ages.
Far deeper into the past, in Amphictyonic League, the cutting of water supply as a war measure was forbidden. It was about mid-7th century BC.