Environment
Novell ZENworks 10 Configuration Management with Support Pack 1 - 10.1
  Situation
 zac unr -f  requires Administrator user name and password even when unregistering locally.   
Resolution
This is fixed in version 10.2 - see KB 7003225 "ZENworks 10 Configuration Management 10.2" which can be found at https://www.novell.com/support 
In 10.2 there is a new -s  option to suppress prompt for Administrator and password when unregistering locally only.
Use:
zac unr -f -s 
To unregister locally and suppress prompt.
  Additional Information
Note:  Unregistering always requires local Windows Administrator rights.  
Using -f  without -s  will unregister from the zone, if available, or locally if not.  Use of -s  implies local only.