I am guessing that you are talking about the GroupByRow. You can hide this row just like you would hide any other row, by setting the Hidden property to true.

However, I suspect that's not really what you want, since this will hide the child rows as well.

There's no way to hide the GroupByRow and still show the children... although, if you don't mind leaving a big space where the GroupByRow used to be, you could probably eliminate the display of the row by using a CreationFilter.