Have you tried editing the instance group to set its size to zero? I believe this will cause the instance group to spin down all the VMs which is what you want to happen.
An error occurred while saving the commentDavid Cardoso commented
I believe this does not just pause the instances, but rather removes them. When you set to a number greater than 0, it creates new instances, rather than starting the same ones from before.
Can anyone confirm this?