To return a package delivered by the U.S. Postal Service, mark it "return to sender" and the postal service sends it back. Return to sender is typically used for items received from companies sending out promotional materials or gifts that the receiver never ordered.
Recipients of unwanted packages are free to do what they want with them. If the package is opened, the receiver is free to keep what is found inside or throw it away. Returning a package received through the United Parcel Service requires a return label. All packages can be returned by UPS by taking the package to a UPS store, dropping it in a UPS drop box or giving it to any driver.