Mine was to work tax policy to incentivize companies to make all their packaging shiny and white, incentivize people to litter, and disincentivize everybody from recycling.
My friend's was to use a giant rocket to push the earth farther away from the sun
Nuclear explosions inside supervolcanos could result in temporary volcanic winter and stop runaway global warming The same could be done by large fires in forests in taiga in Siberia.
But injecting water in the upper stratosphere may be cheapest solution as it will result in small but reflective ice clouds.
I doubt you could set off a supervolcano with such a method, but if you did, you would have a lot more to worry about than global warming.