Write "Not at this address" on each envelope, take it to the PO counter, and hand it to a postal employee with an air of mild exasperation.
There's a university just across the road from where I work. Every year, inevitably, some college student mistakenly enters the wrong PO box number and their mail comes to us. I drive past the place every day, so often I'll stop in and leave the mail item at the campus PO desk. Not my bag, granted, just out of courtesy. Occasionally, I carry it down a couple blocks to the PO, and point out to the person at the desk that the mail is obviously intended for the university. I even correct the address on the envelope with a pen.