You can definitely turn off the sync, because I did it, but I don't remember how. I think you can only see the option while the reader is connected. Sadly, I can't do it right now because my reader is sitting a bowl of rice for a few days (but that's another post.)
Here is another post about it, try this:
https://www.mobileread.com/forums/showthread.php?t=61588
eP