Assuming there is always somebody on the controls, if you are going too fast, then you back off on the throttle. This applies whether going uphill, downhill, on the flat.
If you are asking whether a series motor can provide braking, then that's a different matter.
Consider an ideal un-energised series motor. Connect a resistor to the terminals and spin it. As there's no current flowing, there's no field, so it generates no voltage, which means no current flows, so it generates no voltage, which means ... This is the self-excitation problem of a series machine.
In the real world, the motor stator iron will have some residual field. This means that it will generate some voltage. If the external resistance is low enough, then sufficient extra current will flow to enhance the field for it to generate more voltage.
A series machine that's designed to be a generator will typically use a hard rather than soft iron stator, to retain a large field, or may even have some permanent magnet material in the stator to provide a starting field.