Purge is the best one, and the hardest to do. But what if...
Well, Ubuntu (and every OS under the sun, probably) offers a setup file to be installed in a removable media, that installs an OS on a machine, and maybe runs a virtual one off a virtual memory space.
What if somebody made an UNINSTALL image? A completely agnostic, rewrite-three-times-zeroes-and-ones wiping tool that takes over and cleans ANY machine whole?
Of course, UEFI probably makes it easier, but any platform that runs off x86 (as in, any run-off-the-mill PC with a BIOS I am familiar with) should be able to get something like it. It would need firmware-level access and maybe a bunch of drivers, but you get what I mean.
So, there's probably an underlying difficulty in doing this that I have no idea, but if somebody cracks it, will make a fortune in wiping services.
If it was easy, somebody would have devised an "un-image" ready to deploy and wipe any machine clean by now, so I guess that would be the ultimate goal? You don't even need to worry about encryption, because you are triple-wiping it as well. Just deleting the encryption key isn't security (at all) enough.
I bet the ransomware folks have tools like this ready to go, and they use it against your will.