Fabio,
There are two ways to shut down the switching mechanism of the LM62421.
1) Bring the EN to GND (< 0.4V)
2) Bring Vin below the UVLO level. On start-up this locks out the switching mechanism until the voltage exceeds 2.65V maximum. But to shot-down the part, Vin needs to drop below 1.7V.
Either of these methods will shut down the switching and voltage regulation mechanisms of the part.
Neither of these methods will cause the voltage to be zero if Vin is still present, as Vin has a path to Vout through the diode. There is no true shutdown with a boost converter.
In order to make Vout = 0V you will need to either remove Vin or put some type of on/ off switch-type mechanism that would either cause Vin to be 0V or would switch off Vout.
I have attached an application report that shows several true shutdown circuits (pages 10 -13).
(Please visit the site to view this file)
Please let me know if you have any further questions.
Richard Elmquist