How to prevent a datacolumn of gridview displaying null

Hi I am in my 2nd week of c# .net and I have a datagrid which has a datasource set to a datatable I retrieved from a SQL database. I added a datacolumn to the datasource called "Selected" as a boolean which is the only column the user can change. ie. A tick box has appeared for each row and the user can select / de-select the row The only problem is the user can select the DBNull state. I have disabled this in the definition of the datacolumn but I don't want the user to even be able to set it to null (grayed tick) Can anyone point me in the right direction Cheers Alan :-)