This circuit (click on image to see it enlarged):
(sorry for the bluriness) will make the robot come down the pole slowly. The robot takes off up the pole when S1 is closed. When S2, a SPDT switch, is thrown by contact with the top of the pole, it takes the battery out of the circut and connects the motors so that they act as generators into a short circuit load. (Note that the motors are not internally shorted, a bad thing.) This provides "dynamic braking" as the robot comes down the pole gently by gravity. It works quite nicely.
When taking the robot off the pole, open S1 then reset S2 to arm the minibot for the next run.