[Users] How to remove storage domain

Hello, oVirt 3.2.1 on f18. I have an FC datacenter where I have several storage domains. I want to remove one storage domain, so I move all its disks to other ones (disk --> move) At the end its state is active, but the "remove" option is greyed out. I can only do "destroy". What to do to have the "delete" option possible? Thanks, Gianluca