Could probably place text or picture or even invisable picture/table over it, but i am sure that this would still be against tos, and would certainatly not recomened anyone to try.
Yes, I started a thread yesterday about someone who just changed the height of the ad in the code by 20px less to hide it.
Best would be that you mail google and ask them. Ofcourse, politely I'm sure they'll answer with in a few days. ~G
Hiding or altering the AdSense codes will breach against Google's TOS and policy. On my research, I've found there are many methods for cheating the AdSense's weakness as I've been wrote here : http://www.angellica2017.com/google-adsense-cheats-and-weakness But, although some of those methods doesn't touch the ads directly, it will be categorized as altering the ads behavior.