As far as preventing a volcanic eruption, I have to say that there really is NO WAY a volcanic eruption can be prevented based on current technology. It is similiar to trying to influence the track of a storm...there is so much energy involved that any attempt on the human scale to prevent it or make a storm move from one place to another is FUTILE. The best we can do is to try and FORECAST an eruption. This can sometimes be done if a network of stations on the volcano is set up. Then by monitoring volcanic-related earthquakes as well as small changes in the TILT of the land and the emission of gases, it is possible to estimate in a rough way the chances of an eruption. Unfortunately, this is not an exact science. It is expensive to maintain and deploy a set of instruments and so this is not often done except for volcanos in populated areas.

VOLCANIC ERUPTIONS: Why do they occur??

Well this is a complex question. Basically, magma rises from depths in the Earth because magma is of lower density than the surrounding rock. Just like a balloon filled with Helium, the low density molten rock RISES in cracks and fissures and eventually gets extruded. Sometimes, because the magma cools, it does not make it all the way up to the surface and instead freezes at depth to form a plutonic (as opposed to volcanic) rock. GRANITE is a good example of a common PLUTONIC ROCK.

Finally, there are many earthquakes that DO NOT INVOLVE migration or eruption of magma. There is a type of EARTHQUAKE called " VOLCANIC TREMOR" that IS invariably associated with volcanic eruptions. This is realated to the flow of molten stuff in the conduit. It is similiar to the VIBRATION that gets set up in a pipe of flowing water. When you turn on the shower or let the water run in the kitchen sink, the flow of water in the pipe actually causes some vibration to get set up. Sometimes you can hear this. When magma flows in cracks, the same sort of thing can happen... and we can pick that signal up on a seismometer. Hence we can look for these signals when we are trying to judge whether an eruption may occur.

I have never heard of any way to stop a volcano, and I don't even think geologists are studying how this can be done.The forces that cause volcanic eruptions are so big and so deep that people can't do much except to predict when they will erupt and get out of the way!

Volcanoes erupt because of gases that are in the magma underneath the volcanoes. Typical volcanic gases are water vapor (H2O) and carbon dioxide (CO2), but there is also some nitrogen gas and sulfer dioxide and others. You can think of these gases like the carbon dioxide in a bottle of Coke. You don't see the carbon dioxide because it is dissolved in the Coke, but when you pour the Coke into a glass the carbon dioxide comes out of the liquid and forms bubbles. In the magma under a volcano the same thing happens, but the reason it happens is that the magma is cooling and turning from molten rock to solid rock made of crystals. When part of the magma becomes rock several miles underground there is less magma left over to hold the gas. Eventually the gas becomes saturated in the magma and comes out of solution, forming bubbles. These bubbles exert a lot of pressure on the walls and roof of the cavern of magma, causing a new crack to form or an old vent to open up. When this happens the gas bubbles expand and force the magma out. You can see evidence of these gases in a lot of lava rock that looks like Swiss Cheese or a sponge from the holes in it. These holes were expanding gas bubbles that caused the magma to erupt.
That was a long and maybe complicated answer, so send more questions if you have any.

Maybe you should consider why it is not possible to stop a volcano. I know that this sounds like a cop-out answer but I think that you have enough information that you can answer your own question. How hot is molten lava? What could you make that could stop a lava flow? Would using a steel plate be enough? How about concrete? How much energy is in a volcanic explosion? I've heard that some explosions are as much as an atomic bomb. Look for information like this.

You should try looking for information on plate tectonics in order to get a good answer. A quick answer would not be really meaningful. Use "plate tectonics" as a search word on the internet. You'll find out that the surface of the planet is made up of "plates" of rock that float on magma. A good analogy is when you heat refried beans (or oatmeal) in a pot and the top of the beans cools and forms a crust over the warm beans underneath. The crust will crack into sections. These sections are like the "plates" floating on magma. These plates move over the magma very very slowly (a couple of centimeters a year in some places). Where ever they rub against each other you get earthquakes. In some places they push up against each other until one plate slips under the other. The one that goes under the other ends up going back down into the magma and melts. Some of the stuff that melts is lighter than the magma and rises (like oil poured into water, it all pushes up to the top and floats). But in this case the other plate blocks the rising melted stuff. This stuff will eventually melt its
way through the plate and burst through as lava.