Is it really digging into and gouging the steerer tube or is it just a scratch?
A gouge might be worrisome. As another suggested, I'd get the star nut below that point and consider lowering the bars so the gouge is in the upper part of the stem clamp. Still, depending on how much it's gouged, I'd be a little bothered every time I rode.
I don't know what can be done for a carbon steerer that is damaged, so consider a new fork if you can afford it.