I think you'll find favourites are stored in current.gpx in the GPX folder in the device memory. Copy this file and temp.gpx to a backup location on your PC and then delete them from the device. This should get rid of previous favourites. Make sure they are also deleted in Basecamp so that you don't inadvertantly reload them. current.gpx and temp.gpx will be recreated when you transfer a fresh route from Basecamp.