[RightWhale]

If you try to "vent" the thing

Large pipes driven into the ground with multi-ton steam bobblers hissing on top. Still seems like a workable plan.

[Paul C. Jesup]

The problem is that the Yellowstone magma chamber is like a rubber balloon with a weight on top of it (the top of the caldera). If you try to releave the pressure, the weight on top, pushing down, causes so much pressure that you have a high risk of losing control of the pressure valve and thus creating a full scale eruption of the caldera itself.