This information is wrong.
Download JassNewGenPack (
http://www.hiveworkshop.com/forums/...456/how-download-install-confige-jngp-160547/) and go to:
UMSWE -> Customize Tile Pathability
Here you can alter the pathability of any tile. You can make it unwalkable, unbuildable, or unflyable.
If you don't want to use JNGP you can create your own pathing map with a image editing program. All you have to do then is import it on the correct path for the terrain tile.
Here is a tutorial if you want to create one yourself (this is more work then simply using JNGP though):
http://world-editor-tutorials.thehelper.net/pathmaps.php