Vampire bats have found themselves the victims of widespread extermination efforts due to fears that growth in bat populations will increase the spread of rabies, a fatal disease to both humans and livestock. While vampire bats can spread disease through their bites, only 0.5 percent of bats carry rabies, and the size of a bat colony doesn't predict its rate of carrying the disease. (Culling a colony could actually even end up increasing its disease rate.)
Plus, poisoning or using explosives to destroy bat colonies doesn't just hurt vampire bats - it hurts other kinds of bats, too, many of which are extremely helpful to farmers because they eat insects (not to mention being a vital part of their ecosystems in general.) Finally, vampire bats might have actually helped some humans in Peru become immune to rabies... suggesting that we still have more to learn about the best way to control this disease.
For more on why bats are awesome and to learn how you can support conservation efforts, check out Bat Conservation International.