$ vmadm update a6334dca-f39f-4a2c-87ff-589093d72f55 vnc_port=-1
Invalid value(s) for: vnc_port
According to the man page vnc_port=-1 should disable VNC access.
As mentioned in IRC channel yesterday this may be due to checking for digits only: https://github.com/joyent/smartos-live/blob/master/src/vm/node_modules/VM.js#L993
OS-1752 / joyent/smartos-live#144 fixed regression with setting vnc_p…
fixed per commit listed above. It's now possible to use -1 for vnc_port again.