Wiki

I'm surprised I can't find a setting in the wiki permissions to prevent viewing the wiki contents. (in Nwiki)

This setting exist for most other activities (database, forums...). It's easy to set up with the "override permissions" set up : Mr. xx can view this page ; Mr. yy cannot.

But I don't find how to do this with nwiki. Is there a solution for this ?

I read the forum and found out many people are talking about groups. But it's not what I'm looking for. I don't want people to get on an empty page, I'd like them to get au message "you have no acces here" ; and installing groups doesn't seem to work when a wiki already existing. Using groups just to prevent reading a wiki would be a complicated way to do something that should be simple ;

I just want to set up a course : some visitors can view the contents of this wiki; some others cannot.

Can somebody tell me why Nwiki doesn't offer this setting ?
And is there a way to do it ?

thanks a lot for your replies