The physical Bitcoins (like those by Casascius) work exactly like this: physical coins containing a private key hidden behind a seal.
Lets say I give you a "physical" BTC worth 1 million BTC. Is there any way for me to prove to you that I have erased the private key to this BTC address?
No, there isn't. Anyone who is caught using physical BTC coins is a sucker who doesn't understand the system. They can be very easily scammed by the very man who created the "physical BTC coin". After all, all I need to do is keep a copy of the private address, and transfer the money sometime after I've "sold" you the BTC.