Build the Wall, Enforce the Law Act of 2018 This bill amends the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 to modify provisions relating to the enforcement of immigration laws. Specifically, the bill prohibits any individual (e.g., an employer) from restricting or interfering with the enforcement of the immigration laws by federal officials. It also denies states or localities (i.e., sanctuary jurisdictions) that fail to cooperate in the enforcement of the immigration laws eligibility for federal funds and law enforcement grants under the Omnibus Crime Control and Safe Streets Act of 1968. The bill amends detainer provisions to authorize the Department of Homeland Security to issue detainers for the arrest of an illegal alien if there is probable cause to believe that the alien is inadmissible or deportable. It also provides qualified immunity to governmental and certain nongovernmental entities that detain an alien, except in cases of mistreatment, and allows a private right of action to relatives of victims of crime committed by an alien released from custody by a state or local governmental entity that failed to honor a detainer. The bill also revises provisions requiring detention of aliens during the pendency of removal proceedings to require indefinite detention of aliens who have committed certain serious crimes; enhances penalties for the criminal offense of reentry after removal; establishes new and specific grounds of inadmissibility and deportability for aliens who are members of designated criminal gangs (consisting of five or more persons); and provides funding for border barriers, technology and other resources at the southern border to prevent illegal entry.