Unlike the Electron/Photon on mesh devices there is no protection diode between the USB connector and VUSB.
So when supplying power both ways you need to be extra careful (others even suggest to keep it mutually exclusive) but grounding EN does not mitigate the potential danger for your USB port in case of back-feeding via VUSB in any way.
What you can do is to manufacture a custom USB cable which disconnects the USB Vcc line.
I’ve 3D printed myself a little insulator inlay for the USB-A connector, but you could just cut the red wire or splice in a Schottky diode to mimic the protection diode on the Electron.