The U.S. Customs Service provides services to trademark owners by enforcing rights at the border.  Customs officials can demand disclosure of the identity of any trademark on imported goods, and under certain circumstances, seize goods bearing infringing marks. Congress has authorized Customs involvement in preventing the importation of counterfeit goods, gray-market goods and goods bearing infringing trademarks. Such goods can be seized at the border. However, what happens to them after seizure depends on which category they fit into. 
Counterfeit goods can be destroyed or given to charity after removal of the trademark. Infringing marks will be sent back to the place of origin, or can enter the U.S. after removal of the mark. Or, upon consent of the U.S. trademark owner, either counterfeit goods or goods bearing infringing marks may enter the U.S.

In order for the Customs Bureau to take action at the border, the U.S. trademark owner will have to record with Customs their federally registered mark (or trade name if used for six months). Additionally, trademark owners typically have to alert Customs of specific counterfeiting or infringing activity.  This is purely a practical matter, since Customs has far too many trademarks to effectively monitor without such assistance.