I don't think you are doing anything wrong. If updates are disabled they should be disabled. Full stop. Sounds like a software glitch.

You say that each time you start an Office app this causes Office to update? Are you allowing the update to continue to completion or are you cancelling the update before completion? If you cancel then it will keep trying to complete the install and this could be the reason why it keeps happening. Allow it to complete and then it might not try to install any more updates. Just a guess.

Most people want updates turned on for security issues and fixes so is there a reason you want updates turned off? If not then leave them turned on.