The plain and simple method to teach Royal Mail about SPAM, is to return it like this, and everyone should do this.
1. Collect all the SPAM together until you are going to post your own legitimate mail.
2. Band it all together, mark it with the lettering listed below, on a plain section of the SPAM, or with a plain piece of paper.
NO DOOR TO DOOR SPAM
STOP SENDING UNADDRESSED SPAM
RETURNED FROM: (STICK ADDRESS/POSTCODE HERE)
I WILL CONTINUE TO RETURN ALL UNADDRESSED MAIL BACK VIA THE POST BOX WITH MY LEGITIMATE MAIL
You can add any other messages you wish, but if everyone did this, just think of the headache it will cause to Royal Mail.
Also any other mail sent via companies who do not go via Royal Mail, just circle the return address and return unopened.