To deactivate a user you must be a superuser of AMS and do the following:
1. Go to Super -> Users
2. Find the user you want to deactivate from the list of users on top left side
3. Remove the access from Area Main on the user and refresh the page with F5 and click on the user again.
4. Now under the access table you will see a red button called Deactivate, click here and the user is deactivated.