Note: Sakai does not destructively delete user data when removing users from a site. Therefore, if you remove a user from your site, and then later reinstate that user, all of the user's activity within the site will remain intact once you add the user back into the course or project.
Select the Site Info tool from the Tool Menu of your site.
Tip: You can remove all users from the site by checking the box at the top of the column right next to the Remove column header. However, be sure that you uncheck yourself so you don't remove your own access!