The switches for the brake lights are little plunger type switches like what turns on the light in a car or refrigerator. When you squeeze the lever, a little plunger pops out completing a circuit. Release the lever and the plunger is pushed in.
There are four options:
1) Tighten the rear brake cable slack in hopes the extra tension will retract the lever with the required force to shut off the brake light. Don't tighten it so tight that it rubs all the time! Do it on the center stand and spin the wheel with your hand tightening the cable a half turn at a time until it gets as tight as possible without grabbing. This may or may not solve the problem but at least the brake will be properly adjusted.
2) Remove the plastic on the handlebars and the brake lever. Push the plunger in and let it pop out a few times with a screwdriver observing the brake light to see if the switch is in need of a new one. If so and you have a warranty, then try for a free replacement or just buy a new one and replace it. I can point you straight to a new one if needed later.
3) Remove the plastic and find the two wires running to that switch and unplug one of them. You will be rendering that switch useless so the light will only come on with the front switch, but everyone uses the front and rear in conjunction so it'll only matter if the scooter is inspected by the state motor vehicle dept.. You'll have to squeeze the front to start the engine too.
4) Take it to a mechanic
The rear brake cable adjustment is done by turning the nut on the end of the brake cable. If you have a rear disc brake then there is no cable:
Cable adjuster nut:
Switches:
Rear (Drum brake)
Front(Disc brake, plunger circled)
