If the "junk" mail has your name and address on it, they are duty bound to deliver it because they have an obligation to the sender who entrust RM to deliver their advertising mail. If you don't want to receive it, your beef is with the sender, not RM.
There are several ways of stopping it: write to the company and ask to be taken off their mailing list, or, put two strikes through your address (do not obliterate it) and mark each letter that you don't want as "unsolicited mail, RTS" (Return to Sender) and put it back in the post box. Your postman will receive it back, take note of the two strikes and return it to sender for you. I worked 20 years for RM and this is how the system works.
The form to stop the junk mail that esme123 is talking about, is for the unaddressed items (flyers) that the postmen deliver along with regular mail.