In Map/Tile storage you have the option to set whether storage tiles or not. I guess if you choose not to store tiles maybe modrana deletes these tiles after you quit.
If you downloaded the tiles of a route for offline navigation you have to store these tiles of course and these tiles are kept on the device (or SD card) until you delete them. It does not matter whether you choose another route or not. This is true for the png-s (image file storage). If you choose sqlite storage method I do not know how to delete specific part of the downloaded tiles (maybe you should edit the sqlite databases).