It was not possible to secure your content by not allowing your entire site or webpages to be iframed. Anyone could iframe your site since you had no protection against it.
You can prevent others from iframing your Google Site. See the provision below:
If someone tries to iframe your site into another Google Site or elsewhere, it will just be a blank iframe. For some of us, this resulted in our iframed content going missing. Just check the box in Manage Site options to reenable for iframed content.