By setting that area of the shadow map to not visible. Not possible in World Editor due to poor design and limitations of the program.
In world editor what you can do is re-calculate shadow maps and it will remove areas such as that and generate them somewhere else. This is the easiest thing to do but not necessarily what you want (shadows will still be on the map, just they will make more sense and not be in the open like circled).
The other approach is a little more hands on. By using a MPQ editor like MPQEdit you can access the war3map.shd file inside the map and either manually clean up that area or you can simply delete it from map archive and it will generate a new clean shadow map on start (if it says shadows are corrupt you will need to use a clean dummy map's war3map.shd file to replace the existing one, I am not sure if it will but just mentioning in case).