Gandalf said the following about the Nazgul: "The power of their master is within them, and they will stand or fall with him." (from memory, not 100% accurate)
This leads me to believe that the Nazgul could only be killed by destroying their rings. This is also supported by the fact that the Nazgul were crushed in the flood at the Fords of Bruinen, but still they got back to Mordor and from there to battle again.
No "spell of protection" on the Witch-King is ever mentioned anywhere, but some consider it a good theory for Witch-King's apparent invulnerability.
But actually, Witch-King wasn't invulnerable in any way. As was said earlier in this thread, the prophecy "no Man shall kill him" was nothing more but a prophecy. A prediction of the future, not a spell (why would Glorfindel cast an invulnerability-spell on the Witch-King anyway?).
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