No way in the GUI to reset the per-VM scale factor to follow the global one.

[Socratis]

If you set (even once) the per-VM scale factor in VM Settings > Display > Screen > Scale Factor, then an ExtraData item is inserted in the .vbox configuration file:

<ExtraDataItem name="GUI/ScaleFactor" value="1.5"/>

Once that setting is there, there's no way to reset it back to follow the global scale factor from the GUI, only from the CLI:

VBoxManage setextradata <uuid|vmname> "GUI/ScaleFactor"

A mechanism is needed, such as a "Reset to Defaults" or "Reset to Global" button, where the ExtraData item is removed, and the VM follows the global settings.