In the UK, there is a six(?) digit code which appears on most addressed junk mail, which is some sort of registration number and links to your address. This is what gets flagged by the mail preference service if you request no junk mail. If you have two numbers for some reason, of course you only get half the junk mail stopped! Even if you remove all name and address information from the cr@p you send them back, if that number appears, however small and insignificant, on the envelope or paperwork therein, the mailer will know the sender, which is why some UK people have stopped junk mailings in this way.