[Thingol]

In Tolkien's writings the spirit (Fëa) is industructable. Sauron, like Saruman, Gandalf, and the Balrog, was a Maiar. The Maiar are capable of taking any form they wish. After the fall of Numenor Sauron's body (Hroa) was destroyed. Because Sauron spent so much of his power on malice and creating the Ring he lost the power to take a fair form forever. Sauron's spirit was incarnated into a hideous form that he was trapped in. When a Maiar incarnates itself it reduces its power (Gandalf, Saruman, Radagast, Alatar, and Pallando voluntarily incarnated themselves). When the Ring was destroyed Sauron lost so much of his power that he was unable to maintain any form. However his spirit was not destroyed, it was weakened to such an extent that it couldn't take shape again and was no longer dangerous. In the Silmarillion it says that Sauron's spirit followed his master's spirit into the void. The void is what exists outside of the Kingdom of Arda (Middle Earth).