I think your use of the word dropbox got you some confusing responses, when in hindsight it is clear you mean the dropdown combo as us geeks would call it. What Manichean was referring to is a remote storage software solution called dropbox which does not apply to you.
The list of authors you get presented with in the authors dropdown is from the authors that exist in your database. So to reduce the list, you have to actually rename those dud authors on books. Once there are no books for a particular author name then it will no longer appear in the dropdown.
Using the tag viewer pane on the left side of the screen in the authors section is a good way to rename incorrect authors in bulk.