How can I prevent a particular cell from being editable

You can do this by deriving a custom column style and overriding its virtual Edit member. Below is an override that will prevent the cell in row 1 of the column from getting the edit focus. You can paste this code in the DataGridDigitsTextBoxColumn sample to see it work.

If you want a more flexible solution, you could expose an event as part of your derived columnstyle that fires right before the call to the baseclass in the Edit override. This would allow the handler of the event to set the enable value depending upon the row and column parameters that are passed as part of the event args. You can download a sample (C#, VB) that implements this technique. The sample also fires the event right before painting the cell to decide whether to paint a gray background for the disabled cell. You could modify the eventargs to include a backcolor, and use this event to color cells based on row and column values.

//this override will prevent the cell in row 1 from getting the edit focus
protected override void Edit(System.Windows.Forms.CurrencyManager source, int rowNum, System.Drawing.Rectangle bounds, bool readOnly, string instantText, bool cellIsVisible)
{
  if(rowNum == 1)
    return;
  base.Edit(source, rowNum, bounds, readOnly, instantText, cellIsVisible);
}