If server responds with 404 not found (or any other error) for non existing tiles CloudGPS handles it correctly. But server can also serve a map filled with junk ('this tile does not exist, try zooming out') and CloudGPS will politely present it as a map to user. CloudGPS just does not care what the tile contents is as long as it is a valid image. Well it would be quite tough to detect if an image is a valid part of one of many kind of maps available or satellite imagery or traffic or weather or whatever else. The problem is not really solvable in general case. But for specific servers it is solvable - maybe some support for that will come in future. As a workaround for now you can try lowering maxZoom value in /opt/cloudgps/providers.ini to prevent going to some zoom level for given provider.