The following per-machine VM extradata settings can be used to change the behavior of the VM selector window in respect of certain VMs:

$ VBoxManage setextradata VM-name
                     property true

property can be any of the following:

GUI/HideDetails

Do not show the VM configuration of a certain VM. The details window will remain just empty if this VM is selected.

GUI/PreventReconfiguration

Do not allow the user to open the Settings dialog for a certain VM.

GUI/PreventSnapshotOperations

Prevent snapshot operations for a VM from the GUI, either at runtime or when the VM is powered off.

GUI/HideFromManager

Hide a certain VM in the VM selector window.

GUI/PreventApplicationUpdate

Disable the automatic update check and hide the corresponding menu item.

Note that these settings do not prevent the user from reconfiguring the VM by using the VBoxManage modifyvm command.