You can disallow certain actions when terminating a VM. To disallow specific actions, use the following command:

$ VBoxManage setextradata VM-name GUI/RestrictedCloseActions property[,property...]

property is one of the following:

SaveState

Do not allow the user to save the VM state when terminating the VM.

Shutdown

Do not allow the user to shutdown the VM by sending the ACPI power-off event to the guest.

PowerOff

Do not allow the user to power off the VM.

PowerOffRestoringSnapshot

Do not allow the user to return to the last snapshot when powering off the VM.

Detach

Do not allow the user to detach from the VM process if the VM was started in separate mode.

This is a per-VM setting. You can specify any combination of properties. If all properties are specified, the VM cannot be shut down.