KillSwitch is an Open Source tool that gives you the ability to kill your internet connection immediately.
To use KillSwitch, double-click on the systray icon and then from the dropdown menu, select the connection you want to kill. You will then need to press the 0 key from your number pad (requires the number lock to be on) to either disable or enable the selected connection. Also, while in use, KillSwitch needs to be minimized; if not, when you close it out it will reopen the internet connection you killed.
Killswitch will be useful when the immediate severing of internet connectivity is required, for instance, if your system is under attack, for example.