You muist draw a filled rectangle with the Form's background color.
You can use either:
Form1.Line(0,0,Form1.Width,Form1.Height,col,BF)
col is the form's background color
or
drwForm1.FillRectangle1(brush.Value,rect.Value)
rect is a RectangleEx object, same as for displaying the Map.
drwForm1 is a DrawerEx object for Form1.
brush is a SolidBrush object with the form's background color.
or
drwForm1.FillRectangle2(brush.Value,0,0,Form1.Width,Form1.Height)
If the map is drawn onto the form's forlayer you can use:
Form1.FErase(0,0,Form1.Width,Form1.Height)
Best regards.