Reboot the Mac in single user mode by pressing or holding down Command + S before boot.
At the prompt we need to mount the drive
% /sbin/mount -uw /
We also need to tell the mac this is the first time we are setting up this computer so it creates an administrator account.
% rm /var/db/.AppleSetupDone
This will not erase any user account that is already on there so relax.
Now go through and setup a new account with a different name. I would keep this account pure so if you need to do something you can use it to alter your normal account.
Once logged in you can go to Users & Groups to change a user’s password, profile, settings, or even access his user directory of files without ever logging into their account. Or worse delete their account.
This trick has been working for years. To disable single user mode you can setup a firmware password, however that usually doesn’t workout in the users favor as they often forget it. One could circumvent that easily by changing RAM chips for older Macs or pulling system battery for newer ones.
Encryption is a more secure way of protecting sensitive data. But if one has physical access to a machine, it really doesn’t take much to break into it. Ehh you know what doing this remotely isn’t much of a challenge either.
Sleep well knowing there really is nothing you can do but to shred and sink your beloved Apple in a remote ocean to make it secure.