Sauron vs. Your Mama - A Discussion of Power in Middle-earth

[obloquy]

We might also extend this to the Nazgul. Tolkien said that their power was primarily in the fear that they projected, so if a character such as Eowyn has in her spirit the power to resist that fear, she may be able to overcome the power of the Nazgul and thus penetrate that barrier created by it.

Maybe. On the other hand, the Dagger of Westernesse may have done the whole job, overcoming completely the protective power of the Witch-King's ring. This idea that a power can be "broken" permanently though the body is not yet slain fits with Saruman's death. It explains why he would ultimately die to a weak creature like Grima, using no supernatural weapon. He maintained the power of his voice, that power that was peculiarly his, but he was otherwise nothing more than his physical presence.