If you are still having the issue you will need to work with the admin for the Google Enterprise account to correct it.
You may have to log the device into an account for the Enterprise administrator to get it to grab the updated/un-enrolled status if the device was offline when it was deprovisioned. In some cases I have seen the OU force enrollment lock the device despite being deprovisioned or retied. In those cases I created an OU for the retired devices with all settings allowed.
The Enrollment Settings and config are stored in the bios.
Swapping memory, erasing, and factory reset will not override it.
AFAIK there is not a way to overwrite the bios without first enabling Dev Mode.
It will just overwrite it with the default bios due to the security measures in place.
This doesn't seem to apply to your situation; are you seeing Enrollment for 'Chrome for meetings' or 'Google Hangout'? If so you will need to swap it to ChromeOS by pressing Ctrl+Alt+H and then selecting NO. It will then reboot into ChromeOS allowing you to login to an account or enable Dev Mode assuming it is properly deprovisioned by the admin.
I am in the process of retiring about 20 devices from my company; ChromeBox/ChromeBook/ChromeBase and Chrome for Meetings(ChromeBox).
Official Response from Google Support on the OU thing.
"For developer mode, make sure that the Forced Re-enrollment policy is set to Disable on the Organizational Unit where the affected Chromebox is located. Then proceed with developer mode."