People keep saying Putin must be allowed to save face. In the face of the continuing atrocities I think that time has passed.
I think it is time for the west to tell Putin any use of Nuclear weapons will result in radiation spreading to NATO countries. That will automatically be taken as a declaration of war by Putin, but not the Russian civilian population, on NATO. This also needs to be broadcast from relay transmitters hanging from weather balloons to the Russian population. Such transmitters would be difficult to eliminate and will stay aloft for many hours.
As he knows the Ukranians have proven his army is next to useless and a small NATO force could destroy the rest of his army within days so he needs to sue for peace now. Nothing less than the total removal of all Russian military and quasi-military personnel from all of the 1991 borders of Ukraine will be acceptable.
Unlike Putin NATO does not murder civilians and will do everything in its power to minimize civilian casualties. However, if NATO is forced into this war, they will destroy one Russian city of similar size for every Ukrainian city destroyed and destroy it to the same extent. Unlike Putin however NATO will give the cities 72 hours for all persons to evacuate.
To prevent Putin from defending said cities ten cities will be told to evacuate for every one to be destroyed. To prove that NATO is serious a military target in each of the named cities will be destroyed.
After the conflict Ukraine and NATO will not occupy any part of Russia.