I was recently bringing back to life a CX4-120 storage array, which had a number of connected hosts registered that were no longer active, I was unable to remove the host using the Unisphere GUI, after invoking the deregister command on the last HBA connection, as the host object remained.
After a short period of scratching my head I found an article by Jason Boche (@jasonboche), who was experiencing the same issue and resolved this by restarting the management server on each storage processor (https:/x.x.x.x/setup) and reconnecting using the Unisphere GUI to which the connected hosts where no longer available.
The article describing the issue and actions in more detail can be found below:
Deans Blog 