What I do is tie a small plastic bag around the handlebar and stem. Just make a simple knot covering the display. I have done this for close to a month now and have not had any problems. I remember reading it on a forum that was dealing with bike security issues. So far it has worked for me because I would disconnect all the wiring from the display and the button that turns on the bafang. The problem I encountered is in the Winter with the connectors are outside they took on a slightly different shape than the connectors I had indoors with me all day. When I would get outside to make the connections with the brakes and display and on switch with the battery, the connectors would not line up. In 30 degree weather trying to connect this without gloves became a challenge. Then I read that in a forum. My problem has been solved.
All I do now is connect the battery (voltmeter is already connected to the battery) with the motor, press the on button and watch the display light up through the black garbage bag. I don't want anyone seeing me take out the bag while the bike is still parked.
Problem solved for now.