On/Off control of step-up DC to DC converter
As you can see on the schematic, with this type of boost converter, there is a direct path from input to output through the inductor and the diode. Even if you remove the IC from the circuit altogether, this path still exists.

If you want the output voltage to go to zero, you'll have to switch the input voltage separately, e.g., using a P-channel MOSFET. The enable pin of the IC isn't going to do it for you.