If you use Calibre, you can install the Kindle Collections plugin (see my signature for the link) which includes a feature to Modify the Justification option on your Kindle. It basically does exactly what PoP described, except it edits the file for you instead of you having to find the right editor to use. You still need to do the restart yourself
Not all books will let you change their justification - you just have to try it and see.