What or who can kill Orochimaru's life force?
Let's get the facts right:


Sasuke did not kill Orochimaru. Orochimaru entered Sasuke's mind and tried to take it over, but failed. Hence he got "sealed" inside.
That's why Orochimaru continued to live inside of Sasuke, after exhausting all of his chakra, Sasuke failed to restrain Orochimaru and he broke free (only for Itachi to permanently seal him again).
Orochimaru's Cursed Seal allows him to inject his snake sage chakra. That allows him (much like Minato and Kushina did with Naruto), to have part of his consciousness sealed inside of the people he gives his cursed mark to.

Due to that fact, Orochimaru's consciousness lives inside of Anko, and after supplying it with some flesh from Kabuto (who contained Orochimaru's and Hashirama's cells), the consciousness part was able to retain control and a body. Also, absorbing all of his own cells from Kabuto, he managed to get the information he needed, as well as his life-force, to recover.


Actually, I'm pretty sure that if he's killed now, he'll die (unless he has some backup copy/cells/whatever hidden elsewhere, waiting for the next sucker to discover and awaken).