Block these and there will be no updates for anything because it won’t be able to check for them:
Stable:
https://s3.amazonaws.com/hassio-version/stable.json
Beta:
https://s3.amazonaws.com/hassio-version/beta.json
You would also want to turn off auto-update for each addon.
You may find yourself in a situation where an update for an addon depends on an update to the supervisor or HA, or an HA update breaks because the supervisor wasn’t updated…there are several possible combinations here, so be mindful.