Currently it is not possible to easily unassign a user from a personal VM. There are a few solutions but they aren’t sysadmin friendly. Please consider to implement an unassign user action/button from...
Seneca_Friend
Microsoft
MicrosoftPersonal desktop unassignment/reassignment is now GA! https://techcommunity.microsoft.com/t5/azure-virtual-desktop/azure-virtual-desktop-support-for-personal-desktop-unassignment/m-p/3168914