Hide the subtitle/caption from all pictures

This is not possible at the moment. Maybe in future updates.

You can put this in your stylesheet to achieve this.
.gallery-item:hover .gallery-caption { opacity: 0!important; }
If you don’t know how to put this in the stylesheet of your theme you can download our plugin Lazyest stylesheet and place it in here.

It is not the best solution but for now it works

The caption is removed only when my mouse moves over the picture. I need it to be removed always.

Could you provide me a link to this?

It’s an internal website, for testing ??

Try this .gallery-caption { opacity: 0!important; }
Or .gallery-caption: {display: none; }