A lockout and tagging device and assembly for a switchable energy isolation device such as a terminal block are provided. The lockout and tagging device includes a base member configured to support a tag and a male member extending from the base member and including a retaining portion complementary to a female receptacle of the isolation device to retain the male member in a female receptacle after insertion of the male member into the female receptacle. The lockout and tagging device also includes a blocking member extending from the base member. The blocking member positively blocks an actuator of the isolation device from moving and switching the isolation device into its non-isolating state when the male member is retained in the female receptacle. A circuit coupled to the isolation device is positively de-energized when the actuator is blocked in the isolating state.