There is no "synchronisation" between the device gallery (I assume you mean the Camera Roll) on the iPhone and the LrMobile app on that phone. There is, however, an auto-add option which ensures that any new photos added to the camera roll are also automatically added to the LrMobile app. If that's what you mean, and you want to stop that from happening, open the LrMobile settings (tap the gear icon top right of the main app page) and tap on the Import tab....there you can enable or disable auto-add of Photos, Screenshots and Videos.
Or do you mean you want to stop the synchronisation between the LrMobile app and the Adobe cloud? That's only possible by pausing sync....tap on the Cloud icon on the LrMobile screen and then tap on Pause Syncing.