Simulate airplane mode in iPhone Simulator

You can use Little Snitch too to temp. block the internet connection for this app.

Turn off your network connection on the Mac.

You could install Apple's Network Link Conditioner

On Yosemite:

Network Link Conditioner can be found in the "Hardware IO Tools for Xcode" package. This can be downloaded from the Apple Developer Downloads page. Once the download has finished, open the DMG and double-click "Network Link Condition.prefPane" to install (source).

With this preferences panel you could create a profile with 0kbps speed.

On older versions of OSX:

Mountain Lion / Mavericks: Xcode > Open Developer Tool > More Developer Tools
Lion: /Developer/Applications/Utilities/Network Link Conditioner