You can override public virtual void DoubleClickRow(int row, bool toggleExpand, int col) of FlyGrid method to avoid node expanding:
[c#]
public class ExtFlyGrid : FlyGrid
{
// ....
public override void DoubleClickRow(int row, bool toggleExpand, int col)
{
if (!toggleExpand)
base.DoubleClickRow(row, toggleExpand, col);
}
}