Although you as the instructor will always see every graded item as a column in the gradebook, you can hide that column from the students until grades are set.
From the course page, on the left side under Administration, click .
In the pull-down menu in the top left corner, under Categories and items, select .
In the Actions column of this table
are a number of icons. Click on the
open-eye icon to hide the column. Click on the
closed-eye icon when you are ready to reveal that grade to
the students.