I will be using for an example in this answer the collapse of the lava dome at Mount St Helens in 1980. The lava dome on top of the mountain grew between March 20 and May 18, and then a landslide caused by the angle of the top of the dome exceeding the angle of repose of the country rock caused the pressure on the magma chamber underneath to be released, causing it to instantly boil and explode. Your suggestion to relieve pressure by either drilling or bombing would have had the same effect. Drilling or bombing a hole in this dome would have caused the pressure to greatly reduce, making all of the dissolved gases in the chamber to exsolve out, causing the pressure to instantly greatly increase, over what the surrounding rock was capable of containing.
So by bombing or drilling a magma chamber that might be about to explode, you run the very real risk of guaranteeing that it will explode. The precursors earthquakes to the Mount St Helens eruption would have been the signal that something was wrong you would use to justify this intervention, and as a result you would have simply advanced the timetable of eruption.
A good analogy is that of a radiator cap on a hot car engine. The coolant inside the radiator might be sitting at 20 or 30°C over its boiling temperature, but because it is under higher pressure due to the heat and being pumped around, it has not boiled. Once that pressure is relieved, it will instantly boil out of the radiator because of the reduction in pressure.