Unless the fan or case came with a switch for the LED in fan its not possible to turn them off... however i have cut the power wires that lead to the LED lights in some fans and they do the trick without damaging the fan...
Unless your case fan or case comes with some sort of switch for LEDS, it's not possible to turn them off. However, if you want to be a risk taker, you can cut the power wire that leads to the LED lights in some fans, and it does the trick without damaging the fan. If you decide to do this, make sure to place electrical tape over the exposed wire, so you don't have a chance of your computer catching a blaze.