Liferay 6.1.1 : Disable simultaneous login from different sessions

Add the below properties in portal-ext.properties

     ##
     ## Live Users
     ##

    #
    # Set this to true to enable tracking via Live Users.
    #
    live.users.enabled=true

    #
    # Set the following to true if users are allowed to have simultaneous logins
    # from different sessions. This property is not used unless the property
    # "live.users.enabled" is set to true.
    #
    auth.simultaneous.logins=false