A dynamo is an electrical generator. Since you're operating a pump using a generator, it's easy to control the pump operation in a variety of ways 1. Disconnect the dynamo from its mechanical source. This will stop the generator from creating an electric current which in turn will stop the pump. Remote operation is kind of difficult but it can be done using a cable system for remote control, or using a wireless control to engage and disengage the dyanamo from its power source using a battery and Latching or non-latching solenoid-driven clutch. 2. Disconnect the dynamo's electrical output to the pump. Wireless or cabled remote control using a relay to engage/disengage the electrical circuit.