Quickest way to "destroy" the contents of a drive?

Solution 1:

The encrypted contents are already useless, unless you have the recovery key.

If someone ELSE has the recovery key, then change it.

From https://derflounder.wordpress.com/2015/02/02/managing-yosemites-filevault-2-with-fdesetup/

You can add or change recovery keys using fdesetup changerecovery. To change to a new personal key, run the following command with root privileges:

fdesetup changerecovery -personal

You’ll be prompted for the password of an existing FileVault 2-enabled user or the existing personal recovery key. Once entered, a new personal recovery key will be generated and displayed. The former personal recovery key will no longer work.