You can mail it back to the State Department with a note saying to cancel it. I don't recommend dropping it off at a CBP station, because Customs is not involved with issuing passports.

UPDATE: Here's what it says on the official passport application form (emphasis mine):

The information you provide regarding your lost or stolen U.S. passport book or U.S. passport card will be placed into our Consular Lost/Stolen
Passport System. This system is designed to prevent the misuse of your lost or stolen U.S. passport book or U.S. passport card. Anyone
using a passport book or passport card reported as lost or stolen may be detained upon entry into the United States. Should you locate the
U.S. passport book or U.S. passport card reported lost or stolen at a later time, report it as found and submit it for cancellation. It has been
invalidated. You may not use that passport book or passport card for travel.