WE use the following
HKCU
Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
REG_Dword
Value: NoClose
Data: 1
https://technet.microsoft.com/en-us/library/cc938263.aspx
This caught us out years ago because in a non persistant environment if you allow the users to shutdown then the desktop ends up in an "Already used" state.
Not fun.
We solved the issue by using that key.
you should end up with Log off> Lock and switch user (greyed out)