"Doubtless," said I, "what it utters is its only stock and store Caught from some unhappy master whom unmerciful Disaster (lines 62-63)
So this is the equivalent of saying, "Oh, that's just the wind." Faced with (and freaked out by) something as weird as a talking bird, our speaker pushes back. He imagines that this bird is like a trained parrot, who has only learned one word because it has only ever heard one word. If that's true, then the natural world isn't trying to destroy him, and this bird is more like a circus act than anything else.
"Get thee back into the tempest and the Night's Plutonian shore! (line 98)
After trying a bunch of questions on this bird, the speaker finally gets fed up with him, and tries to throw him out. Maybe not too surprising, but check out the way he does it. He tells him to head back out into the storm, and to "the Night's Plutonian shore." He's trying to force the bird back into the chaos of the natural world. His mistake was to open the window in the first place, to let the outside in. Now he's trying to fix that by sending the outside back out again. He's also decided that the stormy night outside is evil, black and hellish, or "Plutonian," as he poetically puts it. Unfortunately, the natural world and all its horror are here to stay. That raven isn't going anywhere.