Radio status is a Zone property. It is on by default in a Zone but If you turn it off in a Zone it should stay off in that Zone.
Is anyone in this thread seeing anything different ?
There is a Feature Request thread for a “default to off” or global off capability.
I’ve set this thread to close in two days so that anyone who is experiencing a bug can respond, but otherwise Feature Request is a more visible place for discussion of changes to existing design than Support.
Edit: Removed a line about group behaviour which is more accurately stated by Noris below. The intended group behaviour is that the group has the Radio status of the first Zone in the group.