I scrape them down, put them through the dishwasher with no soap, and then re-wax them. I only bother to do this if they are really nasty though. Otherwise, I just give them back to the bees to clean up. Usually, if you put the plastic back in a frame, that takes care of any warping. If they are so warped that a frame doesn't take care of it, perhaps they are a lost cause.