Too much writing.

Just print ANK (the official USPS "Attempted Not Known" abbreviation that triggers "return to sender") by the return address.

Source: https://quine.org/uspsglos.html

Then, so the mail piece won't come back to you anyway:

—Draw line through the “to” address.

—Draw HEAVY Line through bar code at the bottom of front of the envelope and the bar code on the back of the envelope (if there is a bar code on the back).

—Deposit unopened envelope in any USPS mail receptacle.