You can alter these permissions, by logging into the admin console, then go to "Communities" and select the space in question. Then click on "Permissions->Community Permissions".
The permissions at this level are more fine-grained than during creation of the space, but this will allow you to alter your settings.
If this applies to the site, edit the top level Space and have the sub-spaces inherit from it (empty check boxes).
Thanks Anthony - i hadn't looked much further than my nose. Working now.