First of all you need to have two or more images selected for the Auto-Sync switch to become active. When you've done that the quickest way to disable it is to press Ctrl+Alt+Shift+A....this works when you're in Library or Develop.
Alternatively, you can use the menu bar: in Library go to Metadata menu and uncheck Enable Auto-Sync. In Develop, the Enable Auto-Sync option is in the Settings menu.
Finally, when in Develop you can click the small switch immediately to the left of the Auto-Sync button at the bottom of the right-hand panel.