How do I stop iTunes from re-syncing to iPhone?
To solve your problem, just simply go into iTunes with your device connected, and click the little icon of a phone below the play/pause button.



(The icon at the bottom right corner of picture)

Once in that page, scroll down to the "Options" and uncheck "Automatically sync when this device is connected.



If the "Sync with this device over Wi-Fi" is on, you should turn this off as well to prevent accidental syncing.

Hope this helps!