This question is only relevant for user-created desktop (created from Desktop UI). Here you can remove all your invited users to make your desktop private again.
Admin-made desktop (created from workspace admin) are never private. They are created with the purpose of sharing with members in the workspace.
The user can see that the desktop is no longer shared by the "share icon" (beside the desktop) disappears.
To make your desktop private again, please, do as follows :
- Navigate to the left sidebar menu
- Click on the Desktop you want to make private again
- Click on the name of your desktop at the top of the screen
- Click Manage Users in the opened menu
- Remove all the users you invited to this desktop