maiar spirits

[Inziladun]

Welcome to the Downs, beorn!

As far as we know from the "canon" books, the only "good" Maia to have his physical body destroyed was Gandalf. It seems clear that he was allowed to return to Middle-earth in a similar (though not identical) form in order to continue his fight against Sauron, and because he had made a conscious choice to sacrifice himself to allow the Fellowship to escape from Moria.

Of the others, Sauron and Saruman were, I think, forced to remain in a kind of purgatorial state; a spiritual form that left them conscious and self-aware, but helpless to affect the physical world. For how long, I don't know. This is merest speculation, but I would suspect that other Maia like the Balrogs and Ungoliant, who had shown evil intent toward the Children of Ilúvatar would suffer the same fate. Ultimately, the Maia, as their greater kin the Valar, were directly answerable to the One, and I think any opportunity for rebodiment would have to meet his approval.

That begs the question, though, of how Sauron was able to gain a new body after being "killed" during the destruction of Númenor. The best answer I have is the Ring: having placed a large portion of his power in an inanimate object, as long as that object existed he would be able to rebody himself, though, as seems to be the case after the end of the Second Age, it took him longer if he did not have the Ring with him.