Paul,
actually XWiki offers quite some fine grained rights administration.
Go to the XWiki administration page and then to 'rights' administration.
There you should see a couple of 'Prevent unregistered users from ...'
options.
Prevent unregistered users from viewing pages, regardless of the page or
space rights no
Prevent unregistered users from editing pages, regardless of the page or
space rights yes
Require unregistered users to solve a captcha when posting a comment on
a page yes
(this is from XE 2.6. but I guess that hasn't changed since then ...)
Actually I'd recommend not to overuse the 'rights' stuff and keep it as
simple as possible.
When you have pages that include other documents hunting for missing
rights can get tricky ...
Andreas