A simple solution is to create an Airplane Mode for your Mac.
Here is how to do this:
- go into Network in System Preferences
- click on Location dropdown menu
- click on plus icon to add a new location
- name the new location 'Airplane Mode' or similar, and click 'Done'
- select the location you just created from the Location dropdown menu
- click on each available network interface in the list at the left of the window in turn, disabling each one
- click on the Configure iPv4 menu, and choose Off
- for Wi-Fi, just click on the Turn Wi-Fi Off button
- click Apply, and this location will block all network activity
When you want to turn networking back on, just select Automatic from the Location dropdown menu, and click Apply
- credit for this solution: http://hints.macworld.com/article.php?story=20130325002258846