At a guess, you are using the KoboTouch driver to send the books to your ereader. If so, you can edit the Save to device template in the driver configuration to remove the author sort directory.
My template is:
Code:
eBooks/{author_sort:sublist(0,1,&)}/{title} - {authors:sublist(0,1,&)}
to remove the author sort directory creation:
Elibrary/{title} - {authors:sublist(0,1,&)}
The :sublist(0,1,&) limits the author/author sort to the first entry in the list which is handy when you have an anthology with 20 or more author names since it keeps the file name manageable.