How can the sevarity of storms be prevented?
There is no known way.
All storms are the result of the heating and cooling of the earth. Right now the El Nino phenomena is being experienced at the equator in the Pacific Ocean. The western U.S. is experience the result of that by a series of unusually heavy rainstorms.
When the tropical waters of the Atlantic between Africa and the Americas rises the frequency and intensity of hurricanes increases.
The one existing theory of how we human might be able to reduce the intensity and frequency of storms is by reducing the amount of greenhouse gases which are introduced into the atmosphere. Heat is the driving force for the worst storms and greenhouse gases by definition increase the amount of heat held in the earth's atmosphere.
There is a controversial practice known as cloud seeding.
Cloud seeding is the process that adds condensation nuclei while also cooling the air to induce precipitation. Usually this is in the form of dry ice. The dry ice is injected into the cloud by rocket or aircraft. The dry ice is much cooler than then the air so the temperature drops. The result is increased condensation and therefore precipitation. They also use things like silver iodide.
When you cloud seed a cloud that normally would not produce precipitation in an area of drought, the benefits are bringing water to where it is needed. The technique can be used in other more controversial ways, although the success is not definitive.
If you seed a cloud early, it is theorized that you can cause precipitation to occur earlier than it normally would. China did this during the 2008 Olympics to prevent rain at the opening and closing ceremonies. End result was no rain at the opening and closing ceremonies, although it is difficult to say if that was entirely because of cloud seeding.
In terms of storms, there has been research and there is plenty of conspiracy theories about cloud seeding hurricanes. The idea is you use cloud seeding to turn a tropical depression into a hurricane. The point of doing this in reducing storm severity is causing the hurricane to lose much of it's energy through precipitation before the storm can really build.
Here is were the controversy comes in. If say the U.S. decides to seed a tropical depression to turn it into a hurricane and have it lose much energy before it reaches the U.S., what about the countries that it goes over on the way, say Haiti for example. It is sort of like giving your bad luck to someone else because you have the science to do it.
Now does cloud seeding work for hurricanes? If you ask many conspiracy theorists they will say yes and that it is going to be used by the military. If you ask actual scientists, they will not say one way or the other because the idea is sound, but there is little proof.