If you want to stop annoying, wastefull junk mail being put through your door, there are three steps to take.
Firstly, stop the unaddressed leaflet stuff by getting a "no junk mail" sticker for your letterbox. Many councils will issue you with a free sticker. For Richmond and Twickenham, for example, just email your name and address to email@example.com to get your free sticker.
Next you need to stop mail that is addressed to you, sent by companies who have bought your address from a database. To do this, you simply register with the Mail Preference Service at http://www.mpsonline.org.uk/mpsr/.
The last and most irritating hurdle is to write to Royal Mail (no, there's no other way!) to ask them to stop sending you the adverts that they have been paid to distribute.
The address is:
Door to Door Opt Outs
Simply tell them: "I would like to opt out of door-to-door deliveries".
Three easy-ish steps and it all stops. REDUCE, re-use, recycle, innit.