Yes.
Press F11 to open the mission editor.
Press F7 to open the terrain terraform editor.
Delete all operations except 0 General.
Click Operation to create a new operation and then pick filter.
Select Filter.
A white box with a line and some red squares will appear. Drag all the squares down to the bottom of the box.
Click apply.
Delete the filter operation (I don't know if this is necessary).
Now it's all flat. Paint the ground so it's all one color, delete the snow and pick a new sky. Save it as newmap.mis, so anytime you feel like making a new map you can work off of it and not have to flatten the map all out again or forget to change the square size.
