You can opt out of junk mail delivered by the Postie, but RM deliberately make it very difficult to find out how to do so, and will generally refuse to understand the question if you ask to have it done. I seem to remember a Postie recently got in hot water for telling people how to do it.
The easiest way I've found is to call RM and ask to speak to somebody who can arrange leaflet deliveries for you, which gets you through to the right department. Once your connected, you can have the conversation you really wanted to have...
I did that myself in a house I was living in 15 years ago and, at that that time, it was implemented by the Postie simply carrying a list of 'forbidden' households. One one occasion, I was awoken by the doorbell at about 7am, to find an apologetic post Lady explaining she'd made a mistake and stuffed some leaflets through, and offering to take it all back.
Seriously though, SS is right, RM make money out of leaflet deliveries and we'd all have to pay more to make up the deficit if the junk stopped coming. Personally, I'd be willing to pay that price but would everybody?