It turns out that you do have a right to prevent delivery of junk mail, thanks to a 1970 court case called Rowan c. Post Office Department (http://scholar.google.com/scholar_case?case=3558098989148411...). In fact, you can download a form 1500 from the US postal service toput this into effect....but you have to do it for each sender of mail, and the form requires you to affirm that you find the amterial 'erotically arousing or sexually provocative,' even though the holding in Rowan is not limited to such cases. Right now the Postal Service redirects consumers to the FTC for help with getting off mailing lists - presumably because a) it does not want the additional hassle and expense of presorting and not delivering junk mail and b) because the Postal Service is famously under financial pressure and the delivery of bulk mail provides a huge chunk of its revenue - bulk mail makes up roughly half of all mail delivered and about 1/3 of USPS revenues (as of 2011 - http://stateimpact.npr.org/new-hampshire/2011/09/27/how-junk...).

I can think of a strategy for forcing the USPS to let consumers opt out of receiving junk mail, but it would involve some expensive and protracted litigation and would likely drive the price of first-class mail up to $1 if implemented, so it would generate a great deal of political opposition. I have not done in-depth research on the legality of the USPS dumping this problem into the lap of the FTC, so my impression that the USPS could be compelled not to deliver junk mail in the first place may be incorrect.