You might try booting in Single User Mode, and then using the command line to setup a new admin account. The monitoring may be superior to just that, but it’s worth a shot.
There are two ways to boot in single user mode, depending on how updated the computer is:
(the new method)
- Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery.
- Select Disk Utility from the Utilities window, then click Continue.
- From the Disk Utility sidebar, select the volume that you're using, then choose File > Mount from the menu bar. (If the volume is already mounted, this option is dimmed.) Then enter your administrator password when prompted.
- Quit Disk Utility.
- Choose Terminal from the Utilities menu in the menu bar.
- You can now enter UNIX commands. When done, choose Apple () menu > Restart.
(the old method)
- Turn on your Mac, then immediately press and hold Command-S to start up in single user mode.
- You can now enter UNIX commands. When done, type “reboot” to restart.
Once in single user mode, prepare to create a new admin account by performing the following commands (no quotes, make sure to include spaces):
- Mount the drive by typing “/sbin/mount -uw /” then enter.
- Remove the Apple Setup Done file by typing “rm /var/db/.AppleSetupDone” then enter.
- Reboot by typing “reboot” then enter.
- Upon reboot, the computer will walk you through the setup process as if the computer was new, having never been set up. Complete the setup process, creating a new admin account.
- Once the new admin account is set up, you should be able to erase the original admin account, including all files and preferences.
I haven’t tried this since the new protocol came out, so hopefully it still works. It’s a handy trick when buying used products. Let me know if it doesn’t work!