Refusing your mail is a very simple two-step process. Take a look at the Domestic Mail Manual (DMM) 508, sections 1.1.1 through 1.1.3, for what the Postal Service has to say about it. I'm giving the details here, but I'm just essentially repeating what the DMM says.

1. Write "Refused" on the mailpiece. I'd suggest doing this near the postage area, although that can be hard to find on some pieces of junk mail. I tend to use a black marker for it to make sure it shows up, but I've used a normal pen and it works just as well. Or, have a rubber stamp made up (they're not very expensive) and use that.
2. Give it back to the Postal Service. Essentially, put it wherever you put the rest of your outgoing mail. I just put it back in my mailbox and raise the flag. If you prefer, you can stick it in a regular blue collection box, or even go into a Post Office and hand it to a clerk there. The mail just needs to get injected back into the mailstream somehow.

Caveats to be aware of

While refusing mail is very easy, there are some details that you need to be sure to be aware of.

• You can only refuse unopened mail. If you open the mail to see what's inside, you need to throw it out or recycle it, since the Postal Service won't take it. By opening it, you accepted the mailpiece. If you want help determining what's junk mail and what isn't before you open it, check out the tips on determining if it's junk.
• Mail that is sent to you as registered, insured, certified, or collect on delivery (COD) generally required your mail carrier to physically give you (or somebody at your address) the mail. If it isn't refused right then, you give up your right to refuse it. Luckily, this isn't generally an issue since usually mail sent that way is actually important and not junk mail.
• If you send somebody a sales promotion, solicitation, announcement, or other advertisement, and they respond to it, you also need to refuse it immediately when offered for delivery, and not afterward. Again, this generally isn't the case.