Garmin Express is designed to automatically keep your device up to date since it is recommended to install any device updates when they are released to keep the device running smoothly. The only way to prevent Garmin Express from auto-syncing with your device would be to disable it from running in the background:
Click Options (the gear icon at the top of the window)
Uncheck Always keep Garmin Express running in the background
Click OK
This will cause Garmin Express to completely shut down when the main window is closed. However, any time you open express, you will still receive the update so I am not sure what this would accomplish.
