There's a directory (folder) called koreader in the kindle's root directory.
First make a copy of it on your computer so you have a backup. Then delete it from the kindle.
Then go into the directory called extensions in the kindle's root directory. You'll see a directory called koreader there too. Make a backup of that and then delete that too.
Double-check to make both directories you deleted are gone.
That's it, as far as I can tell
