46 U.S.C. § 60108

Clearance may be refused to a vessel or vehicle transporting cargo destined for a domestic or foreign port when the owner, master, or other individual in charge refuses to accept cargo tendered in good condition, with proper charges, for the same or an intermediate port by a citizen of the United States. This section does not apply if the vessel or vehicle is already fully loaded (giving appropriate consideration to its proper loading) or is not adaptable to transport the tendered cargo.