speed control PWM, DC motor, L298n with LPCxpresso 1549
As the braking force is a function of the turning speed, you'll probably seen not too much of a difference between braking and simply turning of when the motor only runs at slow speeds. But that will definitely the fun part figuring out what works best, you might even try a short reverse impulse, depending of the driven mass.

You probably want the PWM frequency to be outside of the audible band, but not too high to struggle with the raise time of your driver. Around 30khz is not uncommon. Beside the noise it can produce the low pass characteristic of your complete system defines the lowest frequency you are able to use. With a complete solar panel attached to a motor plus gearbox you won't have any problems with something like 100hz, beside the terrifying sound of it.