How to isolate apps from the internet?
No, you can't without additional software. The only solution would be to cut the Internet connexion of your computer.

You can block incoming connexions though, using the standard Mac OS X Firewall (accessible in the Security &amp; Privacy panel of the System Preferences). When activated, when an application wants to start listening on a certain port, you are prompted to allow or deny the listening.

With additional software, you can use the excellent Little Snitch application that allows blocking outgoing connexions as well.