I just used sandpaper. Whatever finish they use on these is pretty thin. Took me maybe 15min to sand the finish off. I think I used 150-200grit. And as far as these stocks go, they are very forgiving. You aren't going to sand away any features anytime soon. Just make sure you take it down evenly and get all the old stuff off.
Edit- And as you can tell, even with a dark stain you still end up with some blonde highlights. Mine is 8 coats of the Minwax. It wouldn't get any darker.