Well if you are going to do this to the robot for next season, you woudn't necessarily need to use a spike. You could use the SPST reley linked by Mercutio, and hook it to a digital IO pin on the controller, like so:
Code:
+5V-----| |------ Hampster
$ \ |
$ \
IO pin--| \-------- Hampster battery
Excuse the ASCII art, I'm too lazy to draw it up properly...