Use Disk Utility to reformat the affected LUNs as "Free Space".
If formatting as "Free Space" does not allow Xsan Admin to label the LUNs, you should use the following Terminal command:
sudo dd if=/dev/zero of=/dev/disk-id bs=1m count=200
...where, disk-id is the device identifier. You can find this identifier by selecting the disk in Disk Utility and then clicking the Info button in the Toolbar.
Xsan Admin should then list the LUNs, and allow them to be labeled as Xsan LUNs.