You can delete all the activities at once or individually from the History page on Forerunner 110 or 210.
To delete activities:
Deleting activities is not required for this fitness watch. These devices are designed to delete older activities once the internal drive is full. This will allow for new activities to be recorded.
*Deleting all activities was added in a software update. Update via Garmin Express if the message in step #7 does not appear.
Garmin Express - http://connect.garmin.com/en-US/start/