Hello,
I don't know if you managed to sort out the buzzer problem, but there is a brake release leaver at the back of the scooter. If it is lifted and stuck in this position, the buzzer will sound whenever the batteries are plugged in and you will be able to push the scooter easily.
If the leaver is pushed down and the brakes are on. If the buzzer still sounds. There is a small switch operated by the brake release leaver. It may be stuck.
The switch can be found by taking off the batteries and turning the scooter onto one side. The switch is at the back end at the top of the electric motor. It is operated by the brake release leaver.
I hope this helps.
Paul.