Just learned about this on another Catholic forum, from the experience of someone who had the unfortunate experience of receiving obscene mail through the U.S. Postal Service.
There is a form you can file with the U.S. Postal Service (Form 1500) to either:
1. Prohibit a sender who has sent you sexually explicit mail from doing so in the future (for this, you must include with the form the explicit mail you received)
2. Request to be placed on a list of persons who do not wish to receive sexually oriented mail. (Only the form is necessary for this.)
Here is a direct link to the postal service form: about.usps.com
I'm not sure that I will fill out this form myself -- it's something that (thankfully) has not been a problem for me. There was one isolated incident, several years ago, when I received a postcard advertisement for a strip club, but that's the only incident I can remember.
But I thought I would provide this information for those who may benefit from it.