Use an SPDT relay, and connect the other relay contact to GND. This will short out the motor when the relay turns off, stopping the motor - maybe faster than you would like. If it is too fast, play around with some low-value resistors in series with the grounded contact.
EDIT: A reverse-biased 1N4001 diode across the motor should also work.
EDIT: These methods assume that you are switching the high side of the motor, i.e., the other motor lead is connected to ground.