Sorry, you can't. Try a different pin. Here's why:
Assuming you have an Uno or Duemilanove:
In the bottom left corner, you see "PB5" and 19. This is your pin 13. Then you can see where the arrow points; the LED is hardwired in to pin 13. Unless you have amazing skill at SMD soldering and you can take off the LED, you're stuck with it. I would just recommend a new pin.
Full schematic is here if you want it:http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf
ATMega is in the bottom right corner, the other microchip is the USB driver, I think.
Are all your other pins occupied?