As this is the ever-highly speculative Science board, I put to you a highly speculative scenario: Are you a bad enough dude/construction company/government agency/super villain to stop a volcano? Can you think of a way to take enough energy out of the equation to make a volcano sputter rather than explode?
If you flood a volcano, it might not defuse it, but you can harness the resulting steam and use it for energy.
Anybody remember that episode of the Transformers where the Decepticons were draining off the heat of the Earth to fill their energon cubes and it got so cold that you could have a snowball fight in the desert in the middle of July?
If you injected a powerful agent to lower the viscosity of the melt, it would decrease explosive potential. You could heat up the melt a lot while simultaneously introducing iron or other heavier elements to lower the silica content, which would also lower the viscosity and decrease likelihood of powerful eruption.
How about sand? Cheap, otherwise useless, and easy-to-access.
Plus, free glass, right?
I have a special porn folder ready for that day.
You could probably earn more than a million bucks by defusing volcanoes.
Someone needs to defuse the Kilauea volcano before it blows its top and kills us all!!