Hello,
Sorry, but I've been away from Confluence for a while and am having to re-learn certain things...
I understand what the confluence-users group is and why it exists. At the moment it is getting added by default to new spaces. I want to remove it so that only named groups and individuals can access our spaces.
If I remember correctly, removing the confluence-users group from a space will not affect the space's access/edit permissions for the other groups and their users that have been added to the space.
But am I correct? I could go ahead and test this on our live instance, but I'm concerned about messing up permissions and upsetting our users. 
Cheers.