If you fool the unit into thinking that you are importing in new points; it will delete all the existing points. Normally I recommend doing this by hitting MENU - SYSTEM CONFIG - DATA TRANSFER - RECEIVE DATA VIA NETWORK - Turn on Waypoints and Routes then hit RCV. The unit will think you are transferring points from another (non-existent) unit and delete all the points/routes as it prepares for new incoming points (that never come).
This is normally the best way to clean them out without clearing all your other settings.
If you wish to clear everything including your settings; hold CLEAR as you power up the unit until it asks you what language you want.