There's currently no easy way to remove the baked-in eyebrows from the default character skin material.

This is a problem when adding custom eyebrows to a character, as it's necessary for the custom eyebrows to completely cover (hide) the baked-in brows. The only other option is to go in and remove the baked-in skin eyebrows with an external image editor.

It's much easier to add features than to removed baked-in features, so it's best to include as few baked-in features with the skin (especially brows) as possible.