[ineedasername]

You can sanction the countries in a variety of ways until they stop. You could deploy counter measures to prevent some things: a targeted strike against a chemical depot, or deploy anti-drone technology. This is not unlike the Student approach to slowing Iranian nuclear weapons development.

You can also go after the people behind the orders to do these things and bring them up on war crimes. I find the former option much more ethically reliable, as the later frequently suffers from the fact that war crimes tribunals are generally not convened against the actions of whoever wins, while the winner is all to eager to punish their enemies even beyond defeat.

But none of these options are perfect. We live in a world where bad people can do awful things and never suffer the consequences. Justice isn't dead, but it has never been equally applied.