Note: Sakai does not destructively delete user data when removing users from project sites. 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.
Go to Site Info.
Select the Site Info tool from the Tool Menu of your site.
Select user(s) to remove.
In the Remove column, check the box in the row for the user(s) you want to remove from your site.
Select Update Participants.
Select Update Participants to remove the user(s).