You can assign very low priority to the tethered connection. Windows will then only use it when all other connections are not available.
- Go to Control Panel
- Network and Internet
- Network and Sharing Center
- Change adapter settings (left column)
- Right click USB connection and select properties
- Select Internet Protocol Version 4
- Click Proprerties
- Click Advanced (don't change other settings and numbers)
- Deselect Automatic metric and type very high number there (9999, for example)
Screenshot of the settings (click for full resolution):