Once the admin user logged into the system, on the left top of the screen admin user will find a list of the menu from that Click on Administration TAB
On the left panel, the admin user will find a list of the folders hierarchy select user administration under that by clicking the user list admin user will get the list of users in the system.
Click the Switch over the icon 
to Active/Inactive user in the system.
Once you click on the 
an pop-up dialog box will appear and this indicates that the respective user is assigned to following collections and roles.
In this case, you need to remove the respective user from associated roles and collection and then click on the switch over icon
Then the user will be successfully Inactive in the system and will be indicated has inactive as shown below.
