As JimA notes, the box does several functions other than just buzz at you. You can't just unplug it - lot's of stuff will stop working. To quiet the buzzer, you need to pull the box, disassemble it, render the buzzer inoperative, reassemble, and reinstall.
As an alternative, you could just wait a few years. The buzzers always fail with age. My '87 124 hasn't made a peep in years.
If anyone wants to trade a noisy buzzer box for a silent one, I'll offer mine up for trade. (My wife leaves the lights on all the time - I *need* a buzzer.)