Maps Are you talking abou the detailed maps from the CD included (city Select, Metroguide, US Roads and Rec, etc). The base map will always be on your unit but they contain little detail. If you are trying to remove all of the City Select data maps, I'm not 100% sure it is possible now that I think about it. You can overwrite the mapset group. Pick a small area not ever to be used and install it.
If you are talking about routes and waypoints, you need to delete them on the GPS. The mapsource on my two GPS units will only append the routes and waypoints. Usually select routes, menu, delete all.