Hi,

The possible solution that I can suggest in order to disable toggling the header checkbox is to set the GridViewCheckBoxColumn.ShouldCheckDataRows property to false and handle the ViewCellFormatting event where you can disable mouse handling of the checkbox: 

        private void radGridView1_ViewCellFormatting(object sender, CellFormattingEventArgs e)
        {
            GridCheckBoxHeaderCellElement cell = e.CellElement as  GridCheckBoxHeaderCellElement;
            if (cell != null)
            {
                cell.CheckBox.ShouldHandleMouseInput = false;
            }
        }

I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik