You can turn it off by replacing your operating system with another Linux distribution. As of 20.04 you also still have the option of removing Snap outright[1], as long as you don't need the software that is only provided via snap.
Although apparently they are considering[2] adding the option to disable auto-updates.
Thanks, I did this on my 20.04 systems and it worked fine. Simply uninstall snapd. I lost LXD (no way I'd use this now anyway if its distributed via snap), and chromium browser (but discovered the far superior Ungoogled version anyway)