Sick of receiving mail for that guy who lived in your apartment four years ago? In most cases, there's a simple solution to this perennial renter's woe: Write "RETURN TO SENDER" or "NOT AT THIS ADDRESS" clearly on the envelope and put it back in the mail. Or to be more specific, your best bet is actually to hand it off to your letter carrier in person; if you drop it into a blue collection box, it might get lost in the pile without a postal worker seeing your note.
But let's say you diligently wrote "RETURN TO SENDER" on the envelope and put it back in the mail and it still boomerangs back to your mailbox a few days later. In that case, you might be suffering from "loop mail," an unfortunate side effect of USPS's automated "Intelligent Mail" barcode system.