the same goes regarding junk mail.
I return ALL my junk mail but what I do is swap the contents of the envelopes.
Now the same theory works here in that the more people who return the junk the quicker the companies realise it is NOT FINANCIALY WORTH THEIR WHILE and stop sending it out.
There is the rather nice part of it which is that the companies get charged for the mail and if the letter is a bit on the heavy side they get charged even more.

SO SEND BACK YOUR JUNK MAIL.