Kingsley would see to [it] that [Azakaban will no longer use Dementors]. The use of Dementors was always a mark of the underlying corruption of the Ministry, as Dumbledore constantly maintained.
-- J.K. Rowling (BLC)
As the new Minister for Magic, Shacklebolt takes on the dangerous task of removing the Dementors from the ancient, evil fortress of Azkaban (BLC). They had been serving as guards there for hundreds of years, a function which allowed the Ministry to maintain some measure of control over the evil creatures. Shacklebolt replaces the Dementor guards with Aurors (DL).
Commentary
Notes
The Ministry dared not remove the Dementors from Azkaban for hundreds of years, nor remove their "food" in the form of prisoners. The fear was that if deprived of Azkaban, the Dementors would be impossible to control, as they are believed to be impossible to destroy. How then did Kingsley remove the Dementors and where did they go?