OK, so I decided to reboot it on the hopes it would do so quickly andc allow LUN deletion. After a few minutes the web interface stopped saying restarting and is no longer accessible. SMB/CIFS access is no longer possible.
It looks like some services are still running because I got an email alert from the backup to an external eSATA drive overnight, but the NAS is for all intents and purposes bricked until I can force a reboot.
Last time it was like this for 3 days before forcing a reboot and a several day RAID parity check/rebuild.
Any way to force it to gracefully die remotely?