Great question! The word "Nevermore" and its use in this poem is particularly crucial to the overall feeling and dread and suspense that Poe creates. Consider how the different meanings of this word are actually dictated by the narrator in the poem, rather than the bird itself. All the raven says is "Nevermore," it is the narrator of the poem who gives that word a meaning and leads himself on into a terrible cycle of depression and darkness.
"Nevermore" first means that the raven will not forsake the speaker; then that there is no relief, and finally that the raven will give him no peace. Consider how this last meaning is dictated by what is said to the raven by the narrator:
"Take thy beak from out my heart, and take thy form from off my door!"
Quote the Raven "Nevermore."
Of course, the fact that "Lenore" and "Nevemore" rhyme, means that with every answer the narrator is haunted with the echo of her name, and he is kept being reminded of her.