I want to avoid that the anyone can spy the thumbnail console. Anyone knows how to do it? Thanks
The screen thumbnail is only visible to users who have the Virtual Machine > Interaction > Console Interaction privilege.
In other words, if you don't have the permissions to open the VM's console, you won't see the thumbnail (it will show up as a sort of "broken image").
Organize your sensitive VMs in folders and assign Roles in such a way that only the operators that need to have the Console Interaction privilege. You may have to create a new role for that and make sure the above-mentioned privilege is disabled for that role.