On MacOSX (Mountain) Lion, there is no way to turn IPv6 off using the Network settings.
If you don't know the exact interface name you want to turn IPv6 off, list all of them using:
networksetup -listallnetworkservices
Most of the time, you can just use the following commands for Ethernet and Wifi interfaces:
networksetup -setv6off Ethernet
networksetup -setv6off Wi-Fi
To enable again, use:
networksetup -setv6on Ethernet
networksetup -setv6on Wi-Fi