Another way to stop this nonsense is to hit them where its hurts...in the pocket.
The reason that we are plagued with direct mail is that its artificially cheap to send the stuff out in the first place. If they had to pay the same prices as we have to when we go to the Post Office, it would soon cease. But if we respond by POST, than they have to pay, not us.
So, carefully take out the contents of envelope number one, remove anything with your identity on it, and put it all in the return envelope of number two. Repeat !
ie....send the Readers Digest stuff back to the AA, the AA's stuff to NEXT, NEXT's stuff to Cotton Traders, etc, etc.
Its fun and will soon start costing companies money. In the mean time it provides work for lots of Royal Mail.