I really like ChromeOS. Unfortunately Google is not very good with QA and new updates to my ChromeOS device make it completely unusable to me. It keeps getting worse with more functionality getting broken. I've had to roll back using recovery media, but the device keeps updating itself automatically.
To disable ChromeOS updates, I have created custom DNS 'A' records on my router:
I configured DHCP scope for my ChromeOS device to use my router as DHCP server.
Now my ChromeOS device reports that ChromeOS is up to date and never tries to upgrade itself.
It's a shame that I have to disable security updates to avoid making my device unusable.