This can be done only by the admin of a wiki site!
You can always change Your Site from status "public" to "private":
if You really want to do so - you will find this menu under
Site_manager / Access Policy : this opens following frame (see inserted image at the bottom).
Please, do not forget:
- the limitations your site will get ,
- the permissions You have to look again ( for forum too!) ,
- the navigation pages ( nav:top, nav:side) available for the unauthorized visitors (or not)
- the new landing page ( "unauthorized:welcome") you will need,
- the "Joining" page you have to consider (apply allowed , password allowed - or not),
- check the little box with the label "Make this Site private:".