You have to find a suitable texture and immport it in suitable path. Like in skin section it's always said where you have to immport it.
Oh, you can also do other ways:
-You can cover the ground where you want buildings to not bebuilt with rock or any other unbuildable stuff.
-If the ground you want to not be buildable is lower ground than the ground where twers are built, you can fill it with water.
-You can make terrain where creeps go Blighted and make towers be buildable only on non-Blighted ground.
-You can set your terrain to have some other, less ugly ground. Like Dalaran Marble, it looks like a white road.