Officially, you can't disable the light, but if you set it to 0, it will be so weak that you can't actually see it except in complete darkness. If I remember correctly, there is a hack that can be installed after you jailbreak the Kindle, and then you can actually disable the LEDs completely.
A quick search found this:
https://www.mobileread.com/forums/sho...d.php?t=200266
Perform at your own risk. I've never jailbroken my Kindle or tested hacks such as this one.