I'm looking to do the exact same thing - I understand I can limit a permission group to view-only, but I want do deny access completely. In our case we have a set of spaces that area curated and maintained by our independent user group association. All registered members of the community will have access to those spaces, but members of our employee user group need to be excluded completely.
Will be following this thread to see what people come up with!
I am also interested in creating an "exclusionary" permissions group. Is this available in newer versions of Jive?