There are several cases where an administrator may need to have the users becoming inactive at specific dates. In this way, you can control the number of active users in your portal and make sure that you won't exceed your limit.
In order to set a deactivate date for a user, log in as Administrator and move to the Users list (1).
Click on an active user to find the option "Deactivate at.." (2).
Enabling this option you will see a calendar to select the inactivation date (3).