Right. You can't ban the technology, but you can out the nations who employ merciless weapons as uncivilized pariahs who deserve coordinated disrespect, isolation, and punitive measures until the practice ends.
Yes, though I think equally, and maybe the more effective aspect of the prohibition against chemical warfare was the general recognition that going down that pathway would be much more awful for everyone than sticking to conventional methods. This is partly why Germany pretty much avoided using them in warfare during WWII: Once the genie was out of the bottle they would lose any advantage obtained in their initial use. (I don't know if it's apocryphal, but apparently Hitler also disliked them from his own experience in WWI)
I don't disagree with your point, but this part highlights how hard this is in practice. When does "the practice end"? When the war is one? When the current offensive ends? When we believe the devices aren't being produced anymore? This immediately reminds me of how Chamberlain treated Hitler.
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.