This is so darn smart, I'm just upset I didn't think of it first.
My friend and fellow JHU Blue Jay, Ray Wang of Constellation Research, has a brilliant system for identifying which companies sell his name to others.
He uses the "Address 2" line to identify the company to which he is giving his info.
So, for example, he'll write "United Airlines-Spam" or "Verizon-Spam"
The Address 2 line is not subject to mail verification, so it always works.
Then, when he sees a piece of mail coming with that on the address, he knows who sold it.
Before he throws it away, he can call the company and give them a choice:
"Take me off your list or get sued. Your call."