Earlier quoted context omitted.
Hah, tried that. The local post office will just deliver it to your mailbox again, and again, and again. My record is 3 times for a real piece of mail addressed to someone who used to live where I live. After that I just recycled it.
You have to cross out or write over the To address on the envelope. If you add some scribbles to a corner of the envelope, yeah, they won't notice and just re-deliver.
Of course I did that: I crossed out the address, and wrote in big black letters "return to sender, addressee not known at this address". That didn't help. I guess I should have used a sharpie to make the To: address illegible to prevent re-delivery!