Ctrl + N will Start the New Mapfile dialog. At that point either Y or N to save changes to the mapfile with the route. The old route will disappear but display of GPS location and track file will be unaffected. As there is no route on the new mapfile, there is no navigation, just display of location and track.
I use this frequently as I no longer use Street Atlas for actual navigation. While on the road, it is used for track recording only. Before the trip I still use it for trip planning, but my main navigation aid is a Garmin 276C on the dash. Don't know why, but Street Atlas is the only program that will receive Garmin USB data from the 276C for more than 30 minutes to an hour. GPSGate and every other program will crash after that period of time, but I have Street Atlas logs for as much as 16 hours.