Iphoto 11 - how do I reset faces/face detection?

I am a new user of iPhoto 11, today I have been using the "faces" feature, adding face names and manually adding face tags to faces that iPhoto didn't pick up as faces.

I would like to start the face recognition over, i now realise that i had missed a step to help identify faces. is there anyway to empty the cork board/re set the faces data base so that iPhoto will start the faces process again from scratch?

If possible I want to avoid deleting all my photos and re importing.

Thanks in advance.