Letters of Marque and Reprisal.[1] This is how the early U.S fought the Barbary pirates in the Barbary Wars[2] in the early days of the republic. This power is actually authorized explicitly in the U.S Constitution.
And abolished since the end of the Napoleonic Wars, if memory serves well. And thus illegal.
It would be better to fight the defacto slavery of the crews (agencies, owners and so on), the abysmal conditions in these crews home countries and sanction the hell out the owners of these ships and the people / companies making a healthy profit from them. But why bother, because you would just end up with more expensive shipping im general and more expensive fish.
The 1856 Paris Declaration, in fact. Also the 1907 Hague Convention. The US was not a signatory to the former, and did not agree to the provision against privateering in the latter, so for the US, at least, privateering is still on the table. Argentina, however, was a signatory to the Paris Declaration. Perhaps they can change their minds, but probably they won't for something like this.
