Netkill

Netkill is a plain and simple software program that can immediately cut off your Ethernet and WiFi connectivity with one click.
It also implements an option for monitoring one or more processes, in order to auto-terminate the Internet connection when they are launched and restore it when they are stopped.

Portable tool runs in systray

There is no setup pack involved, which makes the app portable. You can save the .exe file in a custom directory on the HDD or a USB flash drive, in order to seamlessly run the program on any computer. However, you may need administrative privileges.
Once deployed, it creates an icon in the taskbar notifications area and gets sent there on close. It silently runs in the background, enabling you to carry on with your normal PC activity without any interruptions.

Kill and restore the Internet connection

Netkill's configuration panel is small and lets you check the current connectivity status, kill the Internet connection with one click, or restore it.
Furthermore, it's possible to create a list with processes to monitor (separated by comma), in order to automatically cut the connection when those applications run, and revive it when they stop.
Lastly, it automatically runs at Windows startup by default, and you can disable this option.

Evaluation and conclusion

The Internet connection was terminated without any issues in our tests when we performed this action manually. No error dialogs popped up and it didn't hang or crash. Unfortunately, the automated method didn't work in our case after creating the process monitor list.
On top of that, Netkill doesn't simplify the network connection disabling mode by integrating a keyboard shortcut or double-clicking its tray icon, for example. Nevertheless, you can test the software application for yourself, since it's free to use.