Hi Stuart,

I confirmed with a Product Manager that the ability to remove an assigned AD user from a device is currently not available.
The current workaround is to either assign the device to a place holder AD user (as you have been doing), or to re-enroll the device.

Please open a new support case and we can help you place a Feature Request.
You can also open a new case using the link provided below:

http://www.soti.net/services/support/supportcase/ 

You may also link to this post to provide the agent with more information:

https://discussions.soti.net/thread/removing-an-assigned-user-from-a-tablet/

Regards, 
~G

Technical Support | SOTI Inc. |1.905.624.9828 | support@soti.net | soti.net |