To unjoin, all the volumes had to go away (except the root/vol0). The Root and any data aggregates may still be around.
The system does a wipeconfig and dumps you back to the Boot Menu.
Choose option 5 to go into Maintenance mode and use the "disk sanitize" command to start sanitizing.
HDD: take a very long time. 4-passes. Three for wipe and one for format.