On a VDA, you might see lots of disconnected session for multiple user accounts by the blank names, that we cannot logoff.

You should be able to use qwinsta/rwinsta for this

qwinsta /server:myxenappserver

Find the session ID you want to kill, then

rwinsta /server:myxenappserver sessionid

Solution 2:
1.)Logon to the VDA, where the ghostuser is showing up and start taskmgr

2.)Go to users > expand user processes and right click "Windows logon application" and select "go to details".
3.)You will go to the winlogon.exe proces in the detail tab for this user. Rightclick winlogon.exe and select "analyze wait chain"
4.)There you will see a svchost proces with a particular PID. Remember that PID

5.)Unfortunately, you can't kill the svchost PID with taskmgr.

Use Process explorer (sysinternals) and run it as admin. Search for the corresponding svchost PID and kill the process.
6.)After that is done the ghostuserlogon will be killed without a reboot.