If you have admin privileges on an App, you may remove users from that App. This does not remove them from the system altogether, only from that particular App.
To remove users from an App:
Click the "Admin" tab.
Click the "App Admin" tab.
You will see a table of Apps. Click on the name of the App you would like to administer.
Click on the "Application User Admin" tab.
You will see a table of users on the Application. For each user you would like to remove from the SC, deselect all privileges for that user (every checkbox for that user's row should be empty). This may be done by clicking the checkbox to the left of the users' name twice. You may also deselect all privileges one at a time.
To save any changes, click the Save Changes button.