1)In command line switch to vbox user:
2)Then follow these instructions:
You can use the command line VBoxManage tool to remove inaccessible VMs. To get a list of VMs run:
The inaccessible ones will show up similar to:
"<inaccessible>" {52a91e62-096d-4f8b-96f4-5fa76db3cd76}
Now you use the GUID in the unregister command: