If you need to reserve (block) some licenses from your Autodesk license server FlexLM/FlexNet for a particular user or a user group, use the option "RESERVE".
In the license folder on your license server, create or edit the text file ADSKFLEX.OPT. Using Notepad.exe, add the reservation lines into this file - e.g.:
RESERVE 1 85768INVNTOR_2012_0F USER usr1
GROUP SUPERCAD usr1 usr2 usr3 usr4 usr5 usr6
RESERVE 4 85871ACD_2013_0F GROUP SUPERCAD
In this example, 1 license of Inventor 2012 is reserved for the user "usr1", and 4 licenses of AutoCAD 2013 are reserved for the group "SUPERCAD" consisting of the users "usr1" to "usr6". You need to restart the license service after you edit the OPT file.
NLM Feature codes can be found on the page Products and on support.autodesk.com.