Common problem. I think there are three possibilities:
1) Install an override switch to turn off battery power to opus.
2) Install a relay to turn off battery power automatically
3) I think Opus can modify the supply to turn off 5v rail.
I don't like the 5v rail staying active either...but I don't want to be without the opus whilst it is modified (its out of the case anyway) so I opted for 1). I shut off the power if I'm leaving the car for any length of time.