I also get a lot of junk mail at my permanent address.  For awhile, instead of recycling all the credit card offers, my dad saved everyone.  He put all the contents of the letter, envelope included, into the return envelope.  He saved for months and then sent them back to the original sender/company.  
In doing this the company not only gets junk mail but they have to pay for the postage.  The idea was that enough people did this it would get too expensive for the companies to send letters out, only to get them back.