I guess the question is what do you do when your opponent flaunts the Geneva convention, but just uses lies to justify them refusing to follow it?

Do you just go "OK fine, if you won't adhere to the GC, we won't agree that your nation is covered by it"?

That just leads to worse outcomes all around. The reality is that there is no real way to enforce this until and unless the offending party is defeated and captured, which is pretty much impossible here.

The appropriate response would be for the rest of the world to look at the totality of Russia's behavior in regard to international law, and to treaties they have signed, and conclude that Russia is a rogue terrorist state that will not adhere to its international agreements, and act accordingly.