It's because in listings the labels use a different class. To apply also in category listings use:

Note that the recommended way to hide labels is to use template overrides, not CSS. CSS is recommended for those who are not familiar with coding and need a quick solution. Details regarding K2 template overrides can be found at getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates .

Good Morning,
first of all thanks for all the previous answers.

I kindly ask you how to hide the label "Extra Fields".
In the specific case please have a look to the test page I'm working on at the following link:

www.ilmondoinmano.eu/contatti-il-mondo-in-mano/testsoggiornix.html

I must remove the label "Informazioni Aggiuntive" leaving only the Extra fields but following the instructions above I can't manage on it.

Thanks for your precious help.