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?
I assume magma probably vaporizes more water than the water cools the magma. So pouring water into a volcano is probably not gonna stop it.
Probably chuck a lot of flowing water over it.
What if you inject the volcano with liquid nitrogen?
>Beowawe Geyser Field was located in central Nevada between Elko and Battle Mountain. The field and steam were visible from the Interstate. The geyser field itself was located on the side of a hill and immediately at the bottom of the slope. This small basin was considered for national monument status, but apparently a political rivalry caused its disapproval. In the 1950s, geothermal drilling began the downward spiral of geyser activity at Beowawe.
One imagines it'd vaporize explosively. BOOM!
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?