You can do this in GUI too. In the FilerView,
click LUNs.Manage
click on the LUN you want to unmap
then click on [Map LUN]
check the Unmap box
click Apply button
Make sure you unmap all the LUNs associated with the igroup. Goto the tree in the FilerView on the left side
click on Initiator Groups.Manage
click on the igroup you want to modify
in the Initiators box, delete the initiator(s)
click Apply button
Then, remap the LU(s) back to the igroup.
Hope that helps,
Wei