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These questions keep coming up. Here's the NHC's answer (the links won't work in my post):

Subject: C5a) Why don't we try to destroy tropical cyclones by seeding them with silver iodide:
Actually for a couple decades NOAA and its predecessor tried to weaken hurricanes by dropping silver iodide - a substance that serves as a effective ice nuclei - into the rainbands of the storms. The STORMFURY project , as it was called, proposed that the silver iodide would enhance the thunderstorms of the rainband by causing the supercooled water to freeze, thus liberating the latent heat of fusion and helping the rainband to grow at the expense of the eyewall. With a weakened convergence to the eyewall, the strong inner core winds would also weaken quite a bit. Neat idea, but it, in the end, had a fatal flaw: there just isn't much supercooled water available in hurricane convection - the buoyancy is fairly small and the updrafts correspondingly small compared to the type one would observe in mid-latitude continental super or multicells. The few times that they did seed and saw a reduction in intensity was undoubtedly due to what is now called "concentric eyewall cycles".

Willoughby et al.(1985)
Concentric eyewall cycles naturally occur in intense tropical cyclones (wind > 50 m/s [100 kt, 115 mph]). As tropical cyclones reach this threshold of intensity, they usually - but not always - have an eyewall and radius of maximum winds that contracts to a very small size, around 10 to 25 km [5 to 15 mi]. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger. A concentric eyewall cycle occurred in Hurricane Andrew (1992) before landfall near Miami: a strong intensity was reached, an outer eyewall formed, this contracted in concert with a pronounced weakening of the storm, and as the outer eyewall completely replaced the original one the hurricane reintensified.
Thus nature accomplishes what NOAA had hoped to do artificially. No wonder that the first few experiments were thought to be successes. To learn about the STORMFURY project read Willoughby et al. (1985). To learn more about concentric eyewall cycles, read Willoughby et al. (1982) and Willoughby (1990).

BTW, other people have asked about nuking the hurricane. Here's the NHC response to that:

Subject: C5c) Why don't we try to destroy tropical cyclones by nuking them ?
During each hurricane season, there always appear suggestions that one should simply use nuclear weapons to try and destroy the storms. Apart from the fact that this might not even alter the storm, this approach neglects the problem that the released radioactive fallout would fairly quickly move with the tradewinds to affect land areas and cause devastating environmental problems. Needless to say, this is not a good idea.
Now for a more rigorous scientific explanation of why this would not be an effective hurricane modification technique. The main difficulty with using explosives to modify hurricanes is the amount of energy required. A fully developed hurricane can release heat energy at a rate of 5 to 20x10¹³ watts and converts less than 10% of the heat into the mechanical energy of the wind. The heat release is equivalent to a 10-megaton nuclear bomb exploding every 20 minutes. According to the 1993 World Almanac, the entire human race used energy at a rate of 10¹³ watts in 1990, a rate less than 20% of the power of a hurricane.
If we think about mechanical energy, the energy at humanity's disposal is closer to the storm's, but the task of focusing even half of the energy on a spot in the middle of a remote ocean would still be formidable. Brute force interference with hurricanes doesn't seem promising.
In addition, an explosive, even a nuclear explosive, produces a shock wave, or pulse of high pressure, that propagates away from the site of the explosion somewhat faster than the speed of sound. Such an event doesn't raise the barometric pressure after the shock has passed because barometric pressure in the atmosphere reflects the weight of the air above the ground. For normal atmospheric pressure, there are about ten metric tons (1000 kilograms per ton) of air bearing down on each square meter of surface. In the strongest hurricanes there are nine. To change a Category 5 hurricane into a Category 2 hurricane you would have to add about a half ton of air for each square meter inside the eye, or a total of a bit more than half a billion (500,000,000) tons for a 20 km radius eye. It's difficult to envision a practical way of moving that much air around.
Attacking weak tropical waves or depressions before they have a chance to grow into hurricanes isn't promising either. About 80 of these disturbances form every year in the Atlantic basin, but only about 5 become hurricanes in a typical year. There is no way to tell in advance which ones will develop. If the energy released in a tropical disturbance were only 10% of that released in a hurricane, it's still a lot of power, so that the hurricane police would need to dim the whole world's lights many times a year.

Now why hasn't somebody or anyone suggested we try to actually stop these hurricanes? There was a project to "super-seed" the hurricane with massive amounts of silver iodide I think to make it basically rain itself out. Now may be the best time to try something.

From Wikipedia...

In the 1960s and 1970s, the United States government attempted to weaken hurricanes in its Project Stormfury by seeding selected storms with silver iodide. It was thought that the seeding would disrupt the storm's eyewall, causing it to collapse and thus reduce the winds. However, it was later discovered that eyewall disruption happens naturally as part of eyewall replacement cycles, and so the success of the program was impossible to gauge.

Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans; covering the ocean in a substance that inhibits evaporation; or blasting the cyclone apart with nuclear weapons. These approaches all suffer from the same flaw: tropical cyclones are simply too large for any of them to be practical.