From the Administration tab, select Users. From here you can Add, Activate and De-Active Additional Users. Keep in mind that only the Administrator or an Alternate Administrator of the account can make changes in this tab.
From here you have the ability to add a new user, de-activate an existing user, re-activate a de-activated user and manger all users permissions.
To de-activate a user, open their profile by clicking their name. At the bottom of their profile is a button to deactivate them. You can reactivate them at any time.
To re-activate a user, click on Show: Deactivated Users.
The user profile will open. At the bottom of their profile is a button to activate them.