If you are in the editor, open the css file (there may be more than one...).
Search in the current file (or all style files) for
Code:
text-align: justify;
and replace with
and do a replace all -- save the search so it's dead easy next time.
That's it for books that come fully justified...except for the odd one with really weird coding.
Now I have a Kindle and a Kobo. If there is no justification specified, the Kindle will do full justification, but the Kobo will not, it gives the ragged-right, which is what I need since I have to read with huge print. So if I'm editing a book and not sure where I'll read it (most books), I want to add explicit left justification to cover the Kindle. I find the common text styles for most paragraphs, and just add it in. (Actually I usually just copy in the whole style coding from my cheater file...standard indent, noindent, chapno, and so on styles. Then I have known and predictable code in the css. I always do things I can at the block level, not the body...call me a control freak.)