If you're using your icons as font, you could set the prefix via the preferences panel (in the font tab). But it won't allow an empty prefix. I will think about changing this behaviour. But for now, you would have to remove the "icon-" from the beginning of your glyph names. There is a textbox to the right of each glyph in the font tab which allows editing names/classes.
But if you're using your icons as SVG, you would have to use the edit/pencil button to edit your icon's tags. The first tag is used for naming. The SVG generator will be improved in future.