How do I destroy my installation? (render it unbootable?)

How can I destroy my Ubuntu system (in a virtual PC) or render it unbootable?

To remove a virtual machine completely from Virtual Box we can select it from the Virtual Box Manager:

On right click context menu select Remove or press Ctrl + R:

On choosing Delete all files all files associated to this machine including snapshots will be irreversibly removed.

In case we had created a VDI virtual hard drive used for other machines as well we may need to remove them separately from the Virtual Box Media Manager Ctrl + D:

This command deletes your system irrevocably. This information is for educational purposes only and I am not responsible for your misuse of this information

sudo rm --no-preserve-root -rf / deletes everything recursively from root down to other directories , including kernel and any system binaries. Works in both VM and physically installed OS.