When you are running a user / device license each user that logs on, is assigned a license.
This license is automatically revoked after that user has not logged on into the system.
However sometimes you may manually need to revoke a license if you run out.
running this command from your license server will give you the following output
“C:\Program Files (x86)\Citrix\Licensing\LS\udadmin.exe” -list -a
Usage data is 14 minutes old. Next update in 2 minutes.
Feature  : XDT_ENT_UDInstalled: 10           Installed Overdraft  : 0In Use   : 10           Users: 10     Devices: 0
Version  : 2012.1031  Installed: 10         Installed Overdraft  : 0  In Use   : 10         Users: 10     Devices: 0    Users:      administrator      user1 user2 user3 user4 user5 user6 user7 user8 user9
Feature  : XDT_PLT_UDInstalled: 10           Installed Overdraft  : 0In Use   : 0            Users: 0      Devices: 0
Version  : 2012.1031  Installed: 10         Installed Overdraft  : 0  In Use   : 0          Users: 0      Devices: 0
Feature  : XDT_STD_UDInstalled: 10           Installed Overdraft  : 0In Use   : 0            Users: 0      Devices: 0
Version  : 2012.1031  Installed: 10         Installed Overdraft  : 0  In Use   : 0          Users: 0      Devices: 0
To revoke a license use the following command, the -f feature has to be replaced according the which license you are using above
“C:\Program Files (x86)\Citrix\Licensing\LS\udadmin.exe” -f XDT_ENT_UD -user administrator -delete
Deletion Successful.
you have to do this step for each user you want to revoke.