Open the sheet, hit the button labeled "Edit" at the top right of the sheet. There should be two sections on the right. One for Players That Can See and the other for Players That Can Edit. There would be an All Players tag under Players That Can See, but if you hover over the tag an X will appear so you can click on the X to remove it from the list. Save it and then no one should be able to see it unless they have GM access or are otherwise listed in that section.